drm/i915/fbc: Move the plane state check into the fbc functions

drm/i915/fbc: Move the plane state check into the fbc functions

Instead of dealing with the presence/absence of the primary
plane in the higher level pre/post plane update code let's
move all that into the fbc code itself. Now the higher level
code doesn't have to think about FBC details anymore.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191213133453.22152-3-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>