drm/amd/display: Remove redundant assignment to variable dc

Smatch report warning as follows:

Line 53679: drivers/gpu/drm/amd/display/dc/core/dc_stream.c:402
dc_stream_set_cursor_position() warn: variable dereferenced before
check 'stream'

The value of 'dc' has been assigned after check whether 'stream' is
NULL. Fix it by remove redundant assignment.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Yi Yang <yiyang13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>