modetest: make use of drmModeRmFB

We should remove the framebuffer before destroying the buffer.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
[Emil Velikov: Tweak the commit message. fb_id = 0 is unused]
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>