intel: Do not assert on unknown chips in drm_intel_decode_context_alloc

There is this long standing nit of igt/tools/intel_error_decode asserting
when you feed it an error state from a GPU the local libdrm does not know
of.

To fix this I need a tweak in drm_intel_decode_context_alloc to make it
not assert but just return NULL (which seems an already possible return
value).

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>