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