drm/i915: move platform_engine_mask to runtime info

If it's modified runtime, it's runtime info.

mock_gem_device() is the only one that modifies it. If that could be
fixed, we wouldn't have to do this.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Maarten Lankhort <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1261406b373998c1a171ee9ed91f7f562826eba6.1660910433.git.jani.nikula@intel.com