drm/i915/gvt: use proper i915_reg_t for calc_index() parameters

drm/i915/gvt: use proper i915_reg_t for calc_index() parameters

In order to be able to use the proper register macros instead of the
underscore prefixed ones, pass i915_reg_t for the calc_index()
parameters.

Side note: DSPSURF is really about planes, not pipes. Fixed stride
doesn't work for plane C for CHV (but that's okay for gvt). This doesn't
support planes beyond C either. But all that is unrelated to the change
at hand.

Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.wang.linux@gmail.com>
Cc: intel-gvt-dev@lists.freedesktop.org
Reviewed-by: Zhi Wang <zhiwang@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/282b19c44d83c96b52c261cfc7218e7e54076cba.1717773890.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>