omap3: allow dynamic selection of gfx_format

Currently, omap3_dss_panel_config() sets gfx_format to a value that is hardcoded
in the code. This forces anyone who wants to use a different gfx_format to make
adjustments after calling omap3_dss_panel_config(). This could be avoided if the
value of gfx_format were parameterized as input for omap3_dss_panel_config().

Make gfx_format a field in struct panel_config, and update existing structs to
set this field to the value that was originally hard coded.

Cc: Wolfgang Denk <wd@denx.de>
Cc: Jeroen Hofstee <jeroen@myspectrum.nl>
Cc: Tom Rini <trini@ti.com>
Cc: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>