drm/i915/display: Add intel_fb_bo_framebuffer_fini

Xe needs intel_fb_bo_framebuffer_fini for taking care of unpinning the fb
and taking reference. In i915 this can be empty.

Also move intel_frontbuffer_get to be done after
intel_fb_bo_framebuffer_init to have reasonable sequences:

intel_fb_bo_framebuffer_init
intel_frontbuffer_get
...
intel_frontbuffer_put
intel_fb_bo_framebuffer_fini

v2: Empty function instead of define

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231207083451.2184562-1-jouni.hogander@intel.com

drm/i915/display: Split i915 specific code away from intel_fb.c

We are preparing for Xe driver. Backing object implementation is differing
between i915 and Xe. Split i915 specific code into separate source file
built only for i915.

v9:
- Use ERR_CAST
v8:
- return original error code from intel_fb_bo_lookup_valid_bo on failure
v7:
- drop #include <drm/drm_plane.h>
- s/user_mode_cmd/mode_cmd/
- Use passed i915 pointer instead of to_i915(obj->base.dev)
v6: Add missing intel_fb_bo.[ch]
v5:
- Keep drm_any_plane_has_format check in intel_fb.c
- Use mode_cmd instead of user_mode_cmd for intel_fb_bo_lookup_valid_bo
v4: Move drm_any_plane_has_format check into intel_fb_bo.c
v3: Fix failure handling in intel_framebuffer_init
v2: Couple of fixes to error value handling

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231203114840.841311-5-jouni.hogander@intel.com