OMAPFB: Clear framebuffers before they are registered

The framebuffers are cleared with the function omapfb_clear_fb(), which
internally calls cfb_fillrect(). The boot logo is copied on to the
framebuffer when the fb device is registered with fb framework.

omapfb_clear_fb() is called after the framebuffer is registered, leading to
the boot logo getting cleared. Clear the framebuffers using omapfb_clear_fb()
before registering the framebuffer devices.

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