drm/radeon: simplify driver data retrieval

You can get the driver data from struct device directly, there's no
need to get the PCI device first.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>