drm/xe: Avoid cryptic message when there's no GuC definition

If there's no GuC firmware entry in the table and the user didn't pass
an override path, the error message is very cryptic: xe will simply try
to continue and then fail when submitting the default context:

	xe 0000:00:02.0: [drm:xe_pci_probe [xe]] XE_LUNARLAKE  64b0:0001 dgfx:0 gfx:Xe2_LPG (20.04) media:Xe2_LPM (20.00) display:no dma_m_s:46 tc:1 gscfi:0
	...
	xe: probe of 0000:00:02.0 failed with error -22

Add an explicit error message and bail out:

	xe 0000:00:02.0: [drm:xe_pci_probe [xe]] XE_LUNARLAKE  64b0:0001 dgfx:0 gfx:Xe2_LPG (20.04) media:Xe2_LPM (20.00) display:no dma_m_s:46 tc:1 gscfi:0
	xe 0000:00:02.0: [drm] *ERROR* No GuC firmware defined for platform
	xe 0000:00:02.0: [drm] *ERROR* GuC init failed with -2
	...
	xe: probe of 0000:00:02.0 failed with error -2

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240201224724.551130-3-lucas.demarchi@intel.com