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>