av1encode: Add "VAEntrypointEncSlice" entrypoint support

Current [av1encode test](https://github.com/intel/libva-utils/blob/master/encode/av1encode.c#L878) always using "VAEntrypointEncSliceLP" , which is giving failure for av1encode test for entrypoint "VAEntrypointEncSlice".

Can you please add support like [h264encode test](https://github.com/intel/libva-utils/blob/master/encode/h264encode.c#L1109)
for av1encode also.
