Some fixes related to unified OpenCL memory

[jenshannoschwalm]

1. The remaining total system memory was wrongfully decreased too far when using unified cl memory possibly leading to decreased performance, also wrong log reports later on.
2. The available memory restrictions due to OpenCL and the resource level are calculated now inside dt_opencl_update_setting(), this is called at startup and when changing preferences so cl_mem setting are always up-to-date and logged there.
3. Checks for available cl_mem when allocating buffers/images have been fixed, relevant for small devices with unified memory.
4. Simplified pipe starting logs, we don't do checks there any more as we do that in (2)