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