drm/radeon/atom: rework encoder enable/disable sequence

This more closely matches what the vbios does and also
adds a quirk for travis lvds displays and powers down
the sink on DP displays which saves some power and may
fix display issues in some cases.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>