drm/i915/frontbuffer: Move bo refcounting intel_frontbuffer_{get,release}()

drm/i915/frontbuffer: Move bo refcounting intel_frontbuffer_{get,release}()

Currently xe's intel_frontbuffer implementation forgets to
hold a reference on the bo. This makes the entire thing
extremely fragile as the cleanup order now depends on bo
references held by other things
(namely intel_fb_bo_framebuffer_fini()).

Move the bo refcounting to intel_frontbuffer_{get,release}()
so that both i915 and xe do this the same way.

I first tried to fix this by having xe do the refcounting
from its intel_bo_set_frontbuffer() implementation
(which is what i915 does currently), but turns out xe's
drm_gem_object_free() can sleep and thus drm_gem_object_put()
isn't safe to call while we hold fb_tracking.lock.

Fixes: 10690b8a49bc ("drm/i915/display: Add intel_fb_bo_framebuffer_fini")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20251003145734.7634-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>