lavc/vaapi_encode: add EQUAL_MULTI_ROWS support for slice structure

lavc/vaapi_encode: add EQUAL_MULTI_ROWS support for slice structure

VA_ENC_SLICE_STRUCTURE_EQUAL_MULTI_ROWS is added to in the latest
libva (1.8.0) which matches the hardware behaviour:

/** \brief Driver supports any number of rows per slice but they must
*    be the same for all slices except for the last one, which must be
*    equal or smaller to the previous slices.
*/

And VA_ENC_SLICE_STRUCTURE_EQUAL_ROWS is kind of deprecated for iHD
since it's somehow introduced in [1] which is misleading from what we
actually handles.

[1]<https://github.com/intel/libva/commit/0e6d5441f19bdc674b4da3169d614d10fd644778>

Signed-off-by: Linjie Fu <linjie.justin.fu@gmail.com>