drm/i915/i915_drv: 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 i915_drv.h file.
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/20230421190026.294208-3-andi.shyti@linux.intel.com