drm/i915: move I915_COLOR_UNEVICTABLE to i915_gem_gtt.h

Declutter i915_drv.h.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230118131538.3558599-6-jani.nikula@intel.com