drm/i915/gvt: Use intel_engine_mask_t for ring mask

When i915 adds additional PVC blitter instances (in an upcoming patch),
the definition of VECS0 will change from bit(10) to bit(18), causing
GVT's R_ALL mask to overflow the u16 storage that's currently used.
Let's replace the u16 with an intel_engine_mask_t to ensure we avoid
this.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220505213812.3979301-8-matthew.d.roper@intel.com