drm/i915/selftest: Remove avoidable init assignment

We can skip the assignment and i915 variable
altogether and use refernce directly. Also used at
single place only.

Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230307094643.532271-1-tejas.upadhyay@intel.com