amd/display: Fix potential null dereference in dce_calcs.c

Reported by smatch:
bw_calcs() error: potential null dereference 'data'

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Ernst Sjöstrand <ernstp@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>