drm/xe/display: stop using intel_fb_obj() in xe_fb_pin.c

drm/xe/display: stop using intel_fb_obj() in xe_fb_pin.c

intel_fb_obj() returns struct drm_i915_gem_object, which is not right
for xe, and only works because xe defines
-Ddrm_i915_gem_object=xe_bo. Switch to intel_fb_bo() and convert to
struct xe_bo from there.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/39f3d52cf156aecc4b2ed5cf1c9342b1c15143b4.1726589119.git.jani.nikula@intel.com