drm/amdgpu: Use ternary operator in `vcn_v1_0_start()`

Remove the boilerplate of declaring a variable and using an if else
statement by using the ternary operator.

Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>