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Currently a FIFO underrun just causes FBC to be deactivated,
and later checks then prevent it from being reactivated. We
can simpify our lives a bit by logically disabling FBC on
FIFO underruns. This avoids the funny intermediate state where
FBC is logically enabled but can't actually be activated.

v2: intel_wait_for_vblank() is no more
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211124113652.22090-16-ville.syrjala@linux.intel.com

Reviewed-by: Mika Kahola <mika.kahola@intel.com>