drm/include: add PP_FEATURE_MASK comments (v3)

Documents PP_FEATURE_MASK enum.
Provides instructions on how to use ppfeaturemasks.

v2: improve enum definitions and add kernel command line parameters to
    ppfeaturemask instructions
v3: fix alignment issues

Signed-off-by: Ryan Taylor <ryan.taylor@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>