drm/omap: fix error handling in omap_framebuffer_create()

When an error happens in omap_framebuffer_create(),
omap_framebuffer_create() calls omap_framebuffer_destroy() if the fb
struct has been allocated. However, that crashes, as
omap_framebuffer_destroy(), which calls drm_framebuffer_cleanup(),
should only be called after drm_framebuffer_init()

Fix this by just calling kfree() for the allocated fb when an error
happens.

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