drm/i915/intel_fb: s/dev_priv/i915/

Rename dev_priv to i915 in the intel_fb.[ch] files.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210325214808.2071517-13-imre.deak@intel.com