drm/i915: fix accidental static variable use

It's supposed to be just a const pointer.

Fixes: 074c77e3

 ("drm/i915/tgl: Gen-12 display loses Yf tiling and legacy CCS support")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191115120440.17883-1-jani.nikula@intel.com