h265parser: Fix short_term_ref_pic_set() size calculation

h265parser: Fix short_term_ref_pic_set() size calculation

This field is used by DXVA/NVDEC/VA, and each specification
describes (NVDEC is not well documented) that it's the number of
bits used in short_term_ref_pic_set().

DXVA doesn't explicitly mention that whether the size of
emulation preventation bytes (EPB) is inclusive or not, but
VA is clearly specifying that it's the size after removing
EPB. Excluding EPB size here makes more sense therefore.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1930>