drm/amd/display: Fix dc_create failure handling and 666 color depths

[Why]
It is possible (but very unlikely) that constructing dc fails
before current_state is created.

We support 666 color depth in some scenarios, but this
isn't handled in get_norm_pix_clk. It uses exactly the
same pixel clock as the 888 case.

[How]
Check for non null current_state before destructing.

Add case for 666 color depth to get_norm_pix_clk to
avoid assertion.

Signed-off-by: Julian Parkin <julian.parkin@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>