drm/i915/fbc: s/dev_priv/i915/

Rename 'dev_priv' to 'i915' to match modern style.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211104144520.22605-16-ville.syrjala@linux.intel.comAcked-by: NJani Nikula <jani.nikula@intel.com>
Reviewed-by: NMika Kahola <mika.kahola@intel.com>