drm/xe/hwmon: Treat hwmon as a per-device concept

There's only one instance of hwmon per device, and MMIO access to it is
always done through the root tile.  The code has been passing around a
pointer to the root tile's primary GT, which is confusing since this
isn't really a GT-level concept.  Replace that pointer with an xe_device
pointer and use xe_root_mmio_gt(xe) to get a pointer when we need to do
register MMIO.  This makes things easier to follow, and also cleans up
the code in preparation for a much larger MMIO register access overhaul
that's coming soon.
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240829220619.789159-6-matthew.d.roper@intel.com