drm/i915: add minimal i915_gem_object_frontbuffer.h

Split out frontbuffer related declarations and static inlines from
gem/i915_gem_object.h into new gem/i915_gem_object_frontbuffer.h.

The main goal is to reduce header interdependencies. With
gem/i915_gem_object.h including display/intel_frontbuffer.h,
modification of the latter causes a whopping 300+ objects to be rebuilt,
while many of the source files actually needing it aren't explicitly
including it at all.

After the change, only 21 objects depend on display/intel_frontbuffer.h,
directly or indirectly.

Cc: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230830085127.2416842-1-jani.nikula@intel.com