drm/xe/display: Fix dummy __i915_inject_probe_error()

When CONFIG_DRM_I915_DEBUG is not set, a dummy
__i915_inject_probe_error() is provided on the xe side. Use the same
logic as in drivers/gpu/drm/i915/i915_utils.c to ifdef it out. This
fixes the build with W=1 and without that config:

	  CC [M]  drivers/gpu/drm/xe/display/ext/i915_utils.o
	../drivers/gpu/drm/xe/display/ext/i915_utils.c:19:5: error: no previous prototype for ‘__i915_inject_probe_error’ [-Werror=missing-prototypes]
	   19 | int __i915_inject_probe_error(struct drm_i915_private *i915, int err,
	      |     ^~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>