drm/i915: Restore the display config after a GPU reset on gen4

On pre-ctg GPU reset also resets the display hardware. Force a mode
restore after the GPU reset, and also re-init clock gating.

v2: Use intel_modeset_init_hw() instead of intel_init_clock_gating()
    in case more relevant stuff gets added there at some point
    Restore interrupts after the reset as well

Tested-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>