drm/amd/display: docstring definitions MAX_SURFACES and MAX_PLANES

MAX_SURFACES and MAX_PLANES now have docstrings that better show the difference between the two.

Reviewed-by: Sun peng Li <sunpeng.li@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>