drm/i915: Report steering details in debugfs

Add a new 'steering' node in each gt's debugfs directory that tells
whether we're using explicit steering for various types of MCR ranges
and, if so, what MMIO ranges it applies to.

We're going to be transitioning away from implicit steering, even for
slice/dss steering soon, so the information reported here will become
increasingly valuable once that happens.

v2:
 - Adding missing 'static' on intel_steering_types[]  (Jose, sparse)
v3:
 - "static const char *" -> "static const char * const" (sparse)

Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220315170250.954380-1-matthew.d.roper@intel.com