drm/amd/display: Assume 32 bpp cursor in DML21

[Why]
Cursor size can change dynamically at runtime without re-validation,
so DML should calculate with the max size cursor to cover all cases.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>