The inline function has no place in i915_drv.h. Move it away, un-inline,
and untangle some header dependencies while at it.

Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220914163514.1837467-1-jani.nikula@intel.com