OMAP: DSS2: Create an enum for DSI pixel formats

Currently, DSI pixel info is only represented by the pixel size in bits using
the pixel_size parameter in omap_dss_device struct's ctrl member.

This is not sufficient information for DSI video mode usage, as two of the
supported formats(RGB666 loosely packed, and RGB888) have the same pixel
container size, but different data_type values for the video mode packet header.

Create enum "omap_dss_dsi_pixel_format" which describes the pixel data format
the panel is configured for. Create helper function dsi_get_pixel_size() which
returns the pixel size of the given pixel format.

Modify functions omapdss_default_get_recommended_bpp() and dss_use_replication()
to use dsi_get_pixel_size().

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>