drm/amd/display: remove dummy is_blanked() to optimise boot time

is_blanked() hook is a dummy one for underlay pipe, hence
when called, it loops for ~300ms at boot.

This patch removes this dummy call and adds missing checks.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>