drm/amd/display: set blank functionality

1. remove the sleep mechanism while set_blank true from
the timing generator.
Since Hw sequencer is the one that manages the flow
he will be responsible for wait for blanck in a critical places.

Signed-off-by: Leon Elazar <leon.elazar@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>