drm/amd/display: Don't ASSERT when total_planes == AMDGPU_MAX_PLANES

[Why]
Can happen on ASICs with 6 planes, but this isn't a bug since we haven't
written outside the array.

[How]
Use <= instead of <.

Cc: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reported-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>