drm/amd/display: Fix disabled cursor on top screen edge

[Why]
The cursor vanishes when touching the top of edge of the screen for
Raven on Linux.

This occurs because the cursor height is not taken into account when
deciding to disable the cursor.

[How]
Factor in the cursor height into the cursor calculations - and mimic
the existing x position calculations.

Fixes: 94a4ffd1 ("drm/amd/display: fix PIP bugs on Dal3")

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>