drm/i915: Add macros to get i915 device from i915_gem_object

We want to stop touching directly i915_gem_object struct members in
intel_frontbuffer code. As a part of this we add helper macro to get i915
device from i915_gem_object.

v2: operate on and return pointer in defined macros

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230727064142.751976-2-jouni.hogander@intel.com