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>