drm/i915: Initialize HWS page address after GPU reset

After GPU reset, HW is losing the address of HWS page in the register.
The page itself is valid except that HW is not aware of its location.

[   64.368623] [drm:gen8_init_common_ring [i915]] *ERROR* HWS Page address = 0x00000000
[   64.368655] [drm:gen8_init_common_ring [i915]] *ERROR* HWS Page address = 0x00000000
[   64.368681] [drm:gen8_init_common_ring [i915]] *ERROR* HWS Page address = 0x00000000
[   64.368704] [drm:gen8_init_common_ring [i915]] *ERROR* HWS Page address = 0x00000000

This patch reloads this value into the register during ring init.

Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>