radeon: handle errors in radeon_hwmon_init()

Smatch complained that the ERR_PTR from hwmon_device_register() wasn't
handled.  I added some error handling in radeon_hwmon_init() to silence
the warning.

Unfortunately errors from radeon_pm_init() aren't handled so this
doesn't really make a difference beyond silencing the warning.

Also I changed DRM_ERROR() to dev_err() which is the new preferred
method.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>