drm/i915/driver: rename driver to i915_drm_driver

As a name, "driver" is too generic and short to be easily located in a
file this size. Rename it to i915_drm_driver.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211111101304.13094-2-jani.nikula@intel.com