drm/i915: Kill most of the FBC register save/restore

We will anyway re-enable FBC normally after resume, so trying to save
and restore the register makes little sense.

We do need to preserve the FBC1 interval bits in FBC_CONTROL since
we only initialize them during driver load, and try to preserve them
after that.

v2: s/I915_HAS_FBC/HAS_FBC/ and fix the check for gen4

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>