drm/xe/psmi: Do not return NULL

drm/xe/psmi: Do not return NULL

The checks for id and bo_size are impossible conditions. If they were
possible, then the caller should not be using IS_ERR(). Just replace
them with asserts which should be compiled out when not debugging and
at the same time prevent other refactors to break this assumption.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/aK1nZjyAF0s7bnHg@stanley.mountain
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250922221133.109921-2-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>