OMAPFB: Adding a check for timings in set_def_mode

When omapfb.mode is passed through bootargs, when omapfb is setting mode,
it would check if timings passed are fine for panel attached to it.
It makes use of check_timing API provided by the panel.

In current code if check_timing API is not available for attached panel,
OMAPFB would return -EINVAL and BPP sent via bootargs will not have any effect.

In case of panels like TAAL panel, omapfb or any other driver should not be allowed to
change the timings. So bpps sent via bootargs will not have an effect.

In such case we can check only the x_res and y_res with the panels resolution
and if they match go ahead and set the bpps.
The bpp value sent via bootarg would have an effect.

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