drm/xe: Fix uninitialized variables

drm/xe: Fix uninitialized variables

Using uninitialized variables leads to undefined behavior.

Moreover, it causes the compiler to complain with:
../drivers/gpu/drm/xe/xe_vm.c:3265:40: error: variable 'vma' is uninitialized when used here [-Werror,-Wuninitialized]
../drivers/gpu/drm/xe/xe_rtp.c:118:36: error: variable 'i' is uninitialized when used here [-Werror,-Wuninitialized]
../drivers/gpu/drm/xe/xe_mocs.c:449:3: error: variable 'flags' is uninitialized when used here [-Werror,-Wuninitialized]

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230523135020.345596-1-michal@hardline.pl
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>