drm/amd/display: Call validate_fbc should_enable_fbc

drm/amd/display: Call validate_fbc should_enable_fbc

validate_fbc never fails a modeset. It's simply used to decide whether
to use FBC or not. Calling it validate_fbc might be confusing to some so
rename it to should_enable_fbc.

With that let's also remove the DC_STATUS return code and return bool
and make enable_fbc a void function since we never check it's return
value and probably never want to anyways.

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