drm/i915/dgfx: Make failure to setup stolen non-fatal

There is no reason to consider the setup of Data Stolen Memory fatal on
dgfx and non-fatal on integrated. Move the debug and error propagation
around so both have the same behavior: non-fatal. Before this change,
loading i915 on a system with TGL + DG2 would result in just TGL
succeeding the initialization (without stolen).

Now loading i915 on the same system with an injected failure in
i915_gem_init_stolen():

	$ dmesg | grep stolen
	i915 0000:00:02.0: [drm] Injected failure, disabling use of stolen memory
	i915 0000:00:02.0: [drm:init_stolen_smem [i915]] Skip stolen region: failed to setup
	i915 0000:03:00.0: [drm] Injected failure, disabling use of stolen memory
	i915 0000:03:00.0: [drm:init_stolen_lmem [i915]] Skip stolen region: failed to setup

Both GPUs are still available:

	$ sudo build/tools/lsgpu
	card1                    Intel Dg2 (Gen12)                 drm:/dev/dri/card1
	└─renderD129                                               drm:/dev/dri/renderD129
	card0                    Intel Tigerlake (Gen12)           drm:/dev/dri/card0
	└─renderD128                                               drm:/dev/dri/renderD128

Reviewed-by: Wayne Boyer <wayne.boyer@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220915-stolen-v2-3-20ff797de047@intel.com