drm/dsi: Add a helper to get bits per pixel of MIPI DSI pixel format

Add a helper that can be used to obtain the number of bits per pixel
corresponding to a given MIPI DSI pixel format. This is useful in
bandwidth calculations, for example.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Chris Zhong <zyw@rock-chips.com>
[treding@nvidia.com: add kerneldoc comment and commit message]
Signed-off-by: Thierry Reding <treding@nvidia.com>