drm/i915: Add getter/setter for i915_gem_object->frontbuffer

Add getter/setter for i915_gem_object->frontbuffer and use it instead of
directly touching i915_gem_object->frontbuffer frontbuffer pointer.

v3:
 - Fix intel_frontbuffer_get return value
 - s/front_ret/cur/
v2: Move getter/setter into i915_gem_object.h

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230727064142.751976-3-jouni.hogander@intel.com