drm/format-helper: Make conversion_buf_size() support sub-byte pixel fmts

There are DRM fourcc formats that have pixels smaller than a byte, but the
conversion_buf_size() function assumes that pixels are a multiple of bytes
and use the struct drm_format_info .cpp field to calculate the dst_pitch.

Instead, calculate it by using the bits per pixel (bpp) and divide it by 8
to account for formats that have sub-byte pixels.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Arthur Grillo <arthurgrillo@riseup.net>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230307215039.346863-1-javierm@redhat.com