drm/i915/guc: Fix a static analysis warning

A static analyser was complaining about not checking for null
pointers. However, the location of the complaint can only be reached
in the first place if said pointer is non-null. Basically, if we are
using a v69 GuC then the descriptor pool is guaranteed to be alocated
at start of day or submission will be disabled with an ENOMEM error.
And if we are using a later GuC that does not use a descriptor pool
then the v69 submission function would not be called. So, not a
possible null at that point in the code.

Hence adding a GEM_BUG_ON(!ptr) to keep the tool happy.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221221193031.687266-3-John.C.Harrison@Intel.com