drm/amd/display: Check pipe->stream before passing it to a function

[WHAT & HOW]
dp_is_128b_132b_signal dereferences pipe->stream so it is necessary to
check it in advance.

Also fix erroneous spaces and move a variable declaration to top.

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>