drm/amd/display: avoid using null object of framebuffer

Instead of using state->fb->obj[0] directly, get object from framebuffer
by calling drm_gem_fb_get_obj() and return error code when object is
null to avoid using null object of framebuffer.

Fixes: 5d945cbc ("drm/amd/display: Create a file dedicated to planes")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 73dd0ad9)