drm/amd/display: Avoid calling blank_stream() twice

drm/amd/display: Avoid calling blank_stream() twice

[Why]
We've made fix for garbage in dcn31_reset_back_end_for_pipe(), adding
blank_stream() before disable_crtc(). And set_dpms_off() will call
blank_stream() again.

[How]
Add flag to avoid calling blank_stream() twice.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Zhongwei Zhang <Zhongwei.Zhang@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>