drm/i915: switch i915_driver_probe() to use i915 local variable

Prefer i915 over dev_priv where possible. No functional changes.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200211162802.16180-2-jani.nikula@intel.com