drm/xe/pcode: Rework error mapping

drm/xe/pcode: Rework error mapping

The sparse array used for error decoding from is unnecessarily big. It
should be better handled by a switch statement that will also allow us
to more easily improve this code.

Add a CASE_ERR() macro to keep the table compact and use it instead of
the 256-entries array, which saves some space:

	$ bloat-o-meter xe_pcode.o.old xe_pcode.o
	add/remove: 0/1 grow/shrink: 2/0 up/down: 190/-4096 (-3906)
	Function                                     old     new   delta
	__pcode_mailbox_rw                           363     465    +102
	__pcode_mailbox_rw.cold                       58     146     +88
	err_decode                                  4096       -   -4096
	Total: Before=7890, After=3984, chg -49.51%

Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Link: https://patch.msgid.link/20251110-pcode-errmap-v2-1-cb18c8f54238@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>