drm/radeon/kms: unpin fb in atombios crtc disable

When drm_helper_disable_unused_functions calls disable
function of the CRTC, it also sets the crtc->fb pointer
to NULL. This can later (when the mode on that CRTC is setup
again from user space) cause ***_do_set_base functions to
"think" that there is no old buffer and skip the unpinning
code. Consequently, the buffer that has been NULL-ified in
drm_helper_disable_unused_functions will never be unpinned
causing a leak in VRAM.

This patch plugs the leak by unpinning the frame buffer
in crtc_disable function.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>