drm/omap: display: don't use dsi_get_pixel_size()

display.c uses dsi_get_pixel_size() which is implemented in the DSI
driver, and we won't have that in the omapdss-base module, to which we
want to move display.c

This patch changes display.c not to use dsi_get_pixel_size(). The call
can be replaced with a simple check for OMAP_DSS_DSI_FMT_RGB565.

We can also make dsi_get_pixel_size() static as it's no longer used
outside dsi.c.

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