media: hantro: HEVC: Fix output frame chroma offset

Hantro decoder doesn't take care of the requested and aligned size
of the capture buffer.
Stop using the bitstream width/height and use capture frame size
stored in the context to get the correct values.

hantro_hevc_chroma_offset() and hantro_hevc_motion_vectors_offset()
are only used in hantro_g2_hevc_dec.c so take the opportunity
to move them here.

fluster HEVC score goes up from 77 to 85 successful tests (over 147)
with this patch.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>