drm/amdgpu/display: add support for LVDS (v5)

This adds support for LVDS displays.

v2: add support for spread spectrum, sink detect
v3: clean up enable_lvds_output
v4: fix up link_detect
v5: remove assert on 888 format

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=105880
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>