drm/xe: Fix uninitialized pointer def

drm/xe: Fix uninitialized pointer def

In the case where a set of checks on xe->info.platform don't assign
a value to pointer def the pointer remains uninitialized and hence
can fail the following !def check. Fix this be ensuring pointer
def is initialized to NULL.

Fixes: 292b1a8a5054 ("drm/xe: Stop ignoring errors from xe_heci_gsc_init()")
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250226160524.566074-1-colin.i.king@gmail.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>