radeon: workaround pinning failure on low ram gpu

GPU with low amount of ram can fails at pinning new framebuffer before
unpinning old one. On such failure, retry with unpinning old one before
pinning new one allowing to work around the issue. This is somewhat
ugly but only affect those old GPU we care about.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org