drm/amd/display: Fix error pointers in amdgpu_dm_crtc_mem_type_changed

The function amdgpu_dm_crtc_mem_type_changed was dereferencing pointers
returned by drm_atomic_get_plane_state without checking for errors. This
could lead to undefined behavior if the function returns an error pointer.

This commit adds checks using IS_ERR to ensure that new_plane_state and
old_plane_state are valid before dereferencing them.

Fixes the below:

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:11486 amdgpu_dm_crtc_mem_type_changed()
error: 'new_plane_state' dereferencing possible ERR_PTR()

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c
    11475 static bool amdgpu_dm_crtc_mem_type_changed(struct drm_device *dev,
    11476                                             struct drm_atomic_state *state,
    11477                                             struct drm_crtc_state *crtc_state)
    11478 {
    11479         struct drm_plane *plane;
    11480         struct drm_plane_state *new_plane_state, *old_plane_state;
    11481
    11482         drm_for_each_plane_mask(plane, dev, crtc_state->plane_mask) {
    11483                 new_plane_state = drm_atomic_get_plane_state(state, plane);
    11484                 old_plane_state = drm_atomic_get_plane_state(state, plane);
                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^ These functions can fail.

    11485
--> 11486                 if (old_plane_state->fb && new_plane_state->fb &&
    11487                     get_mem_type(old_plane_state->fb) != get_mem_type(new_plane_state->fb))
    11488                         return true;
    11489         }
    11490
    11491         return false;
    11492 }

Fixes: 4caacd16 ("drm/amd/display: Do not elevate mem_type change to full update")
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>