drm/amd/display: fix memleak in aconnector->timing_requested

[Why]
when amdgpu_dm_update_connector_after_detect is called
two times successively with valid sink, memory allocated of
aconnector->timing_requested for the first call is not free.
this causes memeleak.

[How]
allocate memory only when aconnector->timing_requested
is null.

Reviewed-by: Qingqing Zhuo <Qingqing.Zhuo@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>