drm/amd/display: use drm defines for MAX CASCADE MASK

[Why]
drm already has this define

[How]
drm Mask is 0x08 vs 0x0800. The reason is because drm mask
works on a byte.        ^^
                 =======||
                 ||
Since the first byte is always zero we can ignore it and only check the
second byte.

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>