drm/i915: Prepare for multiple GTs

On a multi-tile platform, each tile has its own registers + GGTT
space, and BAR 0 is extended to cover all of them.

Up to four GTs are supported in i915->gt[], with slot zero
shadowing the existing i915->gt0 to enable source compatibility
with legacy driver paths. A for_each_gt macro is added to iterate
over the GTs and will be used by upcoming patches that convert
various parts of the driver to be multi-gt aware.

Only the primary/root tile is initialized for now; the other
tiles will be detected and plugged in by future patches once the
necessary infrastructure is in place to handle them.

Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220318233938.149744-4-andi.shyti@linux.intel.com