Fix page flipping when there are disabled CRTCs

Don't call drmPageFlip on a disabled CRTC.

Note that this change is purely the chunk which was missing from
022833e1b8a55f60ee8ededd6b5e37f7d2235c17.

Change-Id: I52ce3fc683eb65d92864c5bbd93dfb5e4408e4ed
1 file changed