modetest: fix the error path handling

Remove the framebuffer and destroy the bo when error occurs on set_mode
and test_page_flip.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
[Emil Velikov: Tweak the commit message.]
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>