drm/i915/gt: Use proper priority enum instead of 0

I915_PRIORITY_NORMAL is 0 so use that instead for better
readability.

Cc: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231023121305.12560-1-nirmoy.das@intel.com