drm/radeon: only apply hdmi bpc pll flags when encoder mode is hdmi

commit 7d5ab3009a8ca777174f6f469277b3922d56fd4b upstream.

May fix display issues with non-HDMI displays.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>