drm/i915: Define multicast registers as a new type

Rather than treating multicast registers as 'i915_reg_t' let's define
them as a completely new type.  This will allow the compiler to help us
make sure we're using multicast-aware functions to operate on multicast
registers.

This plan does break down a bit in places where we're just maintaining
heterogeneous lists of registers (e.g., various MMIO whitelists used by
perf, GVT, etc.) rather than performing reads/writes.  We only really
care about the offset in those cases, so for now we can "cast" the
registers as non-MCR, leaving us with a list of i915_reg_t's, but we may
want to look for better ways to store mixed collections of i915_reg_t
and i915_mcr_reg_t in the future.

v2:
 - Add TLB invalidation registers
v3:
 - Make type checking of i915_mmio_reg_offset() stricter.  It will
   accept either i915_reg_t or i915_mcr_reg_t, but will now raise a
   compile error if any other type is passed, even if that type contains
   a 'reg' field.  (Jani)
 - Drop a ton of GVT changes; allowing i915_mmio_reg_offset() to take
   either an i915_reg_t or an i915_mcr_reg_t means that the huge lists
   of MMIO_D*() macros used in GVT will continue to work without
   modification.  We need only make changes to structures that have an
   explicit i915_reg_t in them now.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221014230239.1023689-13-matthew.d.roper@intel.com