radeon: Call pci_clear_master() instead of open-coding it.

Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>