drm/i915/gvt: move struct engine_mmio to mmio_context.c

struct engine_mmio is not used outside of mmio_context.c. Hide it, and
reduce includes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230926121904.499888-3-jani.nikula@intel.com