drm/xe/device_sysfs: switch over to devm

When hotunplugging the device we currently hit:

kernfs: can not remove 'vram_d3cold_threshold', no directory

Due to directory already being removed. Using drmm for sysfs device
stuff like vram_d3cold_threshold doesn't really make sense when you
consider hot unplugging of the pci device. In such a case the device is
removed however the drmm release action will only fire once the driver
instance can be released, which can be long after the device has already
been hotunplugged. Rather use devm which is directly tied to the device
and should be called when it is removed.

Closes: drm/xe/kernel#1432


Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>