drm/i915/gt: Add general DSS steering iterator to intel_gt_mcr

Although all DSS belong to a single pool on Xe_HP platforms (i.e.,
they're not organized into slices from a topology point of view), we do
still need to pass 'group' and 'instance' targets when steering register
accesses to a specific instance of a per-DSS multicast register.  The
rules for how to determine group and instance IDs (which previously used
legacy terms "slice" and "subslice") varies by platform.  Some platforms
determine steering by gslice membership, some platforms by cslice
membership, and future platforms may have other rules.

Since looping over each DSS and performing steered unicast register
accesses is a relatively common pattern, let's add a dedicated iteration
macro to handle this (and replace the platform-specific "instdone" loop
we were using previously.  This will avoid the calling code needing to
figure out the details about how to obtain steering IDs for a specific
DSS.

Most of the places where we use this new loop are in the GPU errorstate
code at the moment, but we do have some additional features coming in
the future that will also need to loop over each DSS and steer some
register accesses accordingly.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220701232006.1016135-1-matthew.d.roper@intel.com