drm/amd/display: Fix no display on Fiji

Allocate memory for the second pipe allocate_mem_input() needs to
be done prior to program pipe front end. It shows sensitive to
Fiji. Failure to do so will cause error in allocate memory 
allocate_mem_input() on the second connected display.

Signed-off-by: Jerry Zuo <Jerry.Zuo@amd.com>
Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>