drm/i915/driver: rename i915_drv.c to i915_driver.c

This is more about trimming i915_drv.h than the renamed
i915_driver.[ch]. Split out i915_driver.[ch] out of i915_drv.h as a
feasible thing to do.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211111101304.13094-1-jani.nikula@intel.com