drivers: video: introduce video_bits_per_pixel(pixelformat) helper

This was present in the form of video_pix_fmt_bpp() inside ST and NXP
drivers, and was returning the number of bytes, which does not allow
support for 10-bits, 4-bits or non-byte aligned video formats.
The helper leverages the VIDEO_PIX_FMT_*_BITS macros.

Signed-off-by: Josuah Demangeon <me@josuah.net>