drm/i915: Make the DERRMR SRM target global GTT

The ring scratch pages don't have a PPGTT mapping, so the DERRM SRM
should target the global GTT instead.

v2: Add MI_SRM_LRM_GLOBAL_GTT define for -fixes

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>