drm/i915: Move the C3 LP write bit setup to gen3_init_clock_gating() for KMS

Move the MI_ARB_STATE MI_ARB_C3_LP_WRITE_ENABLE setup to
gen3_init_clock_gating() from i915_gem_load() when KMS is enabled. Leave
it in i915_gem_load() for the UMS case, but add an explcit check, just
to make it easier to spot it when we eventually rip out UMS support.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>