drm/i915: Convert i915_gem_flush_ggtt_writes to intel_gt

Having introduced struct intel_gt (named the anonymous structure in i915)
we can start using it to compartmentalize our code better. It makes more
sense logically to have the code internally like this and it will also
help with future split between gt and display in i915.

v2:
 * Keep ggtt flush before fb obj flush. (Chris)

v3:
 * Fix refactoring fail.
 * Always flush ggtt writes. (Chris)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-23-tvrtko.ursulin@linux.intel.com