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>

drm/xe/display: Implement display support

As for display, the intent is to share the display code with the i915
driver so that there is maximum reuse there.

We do this by recompiling i915/display code twice.
Now that i915 has been adapted to support the Xe build, we can add
the xe/display support.

This initial work is a collaboration of many people and unfortunately
this squashed patch won't fully honor the proper credits.
But let's try to add a few from the squashed patches:

Co-developed-by: Matthew Brost <matthew.brost@intel.com>
Co-developed-by: Jani Nikula <jani.nikula@intel.com>
Co-developed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Co-developed-by: Matt Roper <matthew.d.roper@intel.com>
Co-developed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Co-developed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Co-developed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>