drm/i915/mocs: use to_gt() instead of direct &i915->gt

Have to give up the const on i915 pointer, but it's not big of a deal
considering non-const i915 gets passed all over the place.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Acked-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/63e644f056c7745eb0e8e165c990c392a38ec85c.1696236329.git.jani.nikula@intel.com