drm/xe/hwmon: Expose power sysfs entries based on firmware support

drm/xe/hwmon: Expose power sysfs entries based on firmware support

Enable hwmon sysfs entries (power_xxx) only when GPU firmware
supports it.
Previously, these entries were created if the MMIO register
was present. Now, we enable based on the data in the register.

v2: Remove a unnecessary comment. (Rodrigo)

Signed-off-by: Karthik Poosa <karthik.poosa@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250529163458.2354509-7-karthik.poosa@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>