drm/amd/display: remove useless check in should_enable_fbc()

It does not make sense to compare a pointer to array element with NULL.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 65d38262

 ("drm/amd/display: fbc state could not reach while enable fbc")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Dembskiy Igor <dii@itb.spb.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>