drm/i915: split out i915_gem.c declarations to i915_gem.h

Declutter i915_drv.h by splitting out the declarations for
i915_gem.[ch].

Add a fixme comment about the rest of the stuff in i915_gem.h that
doesn't really belong there.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/82a353c5c4b52df2354f9413b547c7619a45d92d.1662390010.git.jani.nikula@intel.com