drm/amd/display: drop ASSERT() if eDP panel is not connected

[Why]
For boards that support eDP but do not have a physical eDP
display connected an ASSERT will be thrown. This is not a
critical failure and shouldn't be treated as such.

[How]
Drop the assertion.

Signed-off-by: Zhan Liu <zhan.liu@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>