drm/radeon/hdmi: make sure we have an afmt block assigned

Prevents a segfault if an afmt block is not assigned to the
encoder such as in the LVDS or eDP case.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=66714

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

diff --cc drivers/gpu/drm/radeon/evergreen_hdmi.c
Simple merge

diff --cc drivers/gpu/drm/radeon/r600_hdmi.c
Simple merge