drm/xe: Fix warning on unreachable statement

eu_type_to_str() relies on -Wswitch to warn (and -Werror) to make sure
it handles all enum values. However it's perfectly legal to pass an int
to that function so in the end that function may happen to return
nothing. There's too much implicit knowledge about the initialization
of eu_type for a compiler  to notice eu_type is never assigned to
anything other than those values.

Trying to reproduce this issue, none of gcc-9, gcc-10 and gcc-13
triggered for me, but this was reported in a different system with
gcc-10:

	drivers/gpu/drm/xe/xe.o: warning: objtool: xe_gt_topology_dump() falls through to next function xe_gt_topology_init()

Also it was reported these warnings when building with clang:

	drivers/gpu/drm/xe/xe.o: warning: objtool: xe_gt_topology_dump+0x77: sibling call from callable instruction with modified stack frame
	drivers/gpu/drm/xe/xe.o: warning: objtool: xe_gt_topology_dump() falls through to next function xe_dss_mask_group_ffs()
	drivers/gpu/drm/xe/xe.o: warning: objtool: xe_gt_topology_dump+0x77: can't find jump dest instruction at .text.xe_gt_topology_dump+0xc0

Since that value is not really possible in real world, just take the
simple approach and return NULL.

Fixes: 7108b4a5 ("drm/xe/uapi: Expose SIMD16 EU mask in topology query")
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240719191534.3845469-1-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>