drm/radeon: make use of radeon_gem_init() consistent

All radeon_gem_init() does is initialize the gem objects
list.  radeon_device.c does this explicitly.  r600+ calls
radeon_gem_init() so the list gets initialized twice.  Older
asics don't call it at all and rely on the the init in
radeon_device.c.  Just call radeon_gem_init() in radeon_device.c
and remove the explicit calls from all the newer asics.

All asics call radeon_gem_fini() in their fini pathes.  That
could possibly be cleaned up too.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>