drm/radeon/si: add missing radeon_bo_unreserve in si_rlc_init() v2

Forget to unreserve after pinning.  This can lead to problems in
soft reset and resume.

v2: rework patch as per Michel's suggestion.

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>