drm/xe/display: fix bo leak when unloading module

One of the remaining issues affecting module unload (DG2 + display)
appears to be some kind of BO leak:

WARNING: CPU: 1 PID: 1198 at drivers/gpu/drm/drm_mm.c:999 drm_mm_takedown+0x2e/0xc0
WARNING: CPU: 1 PID: 1198 at drivers/gpu/drm/drm_buddy.c:174 drm_buddy_fini+0x5e/0x70 [drm_buddy]

Turning on more debugging points at:

[drm:drm_mm_takedown] *ERROR* node [00e10000 + 00e11000]: inserted at
  drm_mm_insert_node_in_range+0x2c8/0x500
  __xe_ggtt_insert_bo_at+0xb0/0x120 [xe]
  xe_bo_create_locked_range+0x255/0x2d0 [xe]
  xe_bo_create_pin_map_at+0x4d/0x170 [xe]
  intel_alloc_initial_plane_obj.isra.0+0x175/0x340 [xe]
  intel_crtc_initial_plane_config+0x7e/0x270 [xe]
  intel_modeset_init_nogem+0x341/0x8b0 [xe]
  xe_display_init_noaccel+0x1e/0x50 [xe]
  xe_device_probe+0x1fa/0x2a0 [xe]

So looks like we are leaking the object ref for the initial fb. Looking
at the i915 code we also drop the driver reference to the underlying BO,
which appears to just leave the drm framebuffer holding the last ref. Do
the same for Xe.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>