drm/i915: split out i915_file_private.h from i915_drv.h

Limit the scope of struct drm_i915_file_private to the files that
actually need it.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e375859dc1729a1b988036e4103e5b1bd48caa00.1644507885.git.jani.nikula@intel.com