drm/amd/display: Fix potential NULL pointer dereferences in 'dcn10_set_output_transfer_func()'

The 'stream' pointer is used in dcn10_set_output_transfer_func() before
the check if 'stream' is NULL.

Fixes the below:
drivers/gpu/drm/amd/amdgpu/../display/dc/hwss/dcn10/dcn10_hwseq.c:1892 dcn10_set_output_transfer_func() warn: variable dereferenced before check 'stream' (see line 1875)

Fixes: ddef02de

 ("drm/amd/display: add null checks before logging")
Cc: Wyatt Wood <wyatt.wood@amd.com>
Cc: Anthony Koo <Anthony.Koo@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>