drm/xe: Clarify 'gt' retrieval for primary tile

There are a bunch of places in the driver where we need to perform
non-GT MMIO against the platform's primary tile (display code, top-level
interrupt enable/disable, driver initialization, etc.).  Rename
'to_gt()' to 'xe_primary_mmio_gt()' to clarify that we're trying to get
a primary MMIO handle for these top-level operations.

In the future we need to move away from xe_gt as the target for MMIO
operations (most of which are completely unrelated to GT).

v2:
 - s/xe_primary_mmio_gt/xe_root_mmio_gt/ for more consistency with how
   we refer to tile 0.  (Lucas)
v3:
 - Tweak comment on xe_root_mmio_gt().  (Lucas)

Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20230601215244.678611-16-matthew.d.roper@intel.com


Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>