drm/i915: Use i915 instead of dev_priv insied the file_priv structure

In the process of renaming all instances of 'dev_priv' to 'i915',
start using 'i915' within the 'drm_i915_file_private' structure.

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230322001611.632321-1-andi.shyti@linux.intel.com