drm/amd/display: Fix calculation for cursor CAB allocation

[Why]
The cursor size (in memory) is currently incorrectly calculated,
resulting not enough CAB being allocated for static screen cursor
in MALL refresh. This results in cursor image corruption.

[How]
Use cursor pitch instead of cursor width when calculating cursor size.
Update num cache lines calculation to use the result of the cursor size
calculation instead of manually recalculating again.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>