Removed bad assert
Removed bad assert

In ModifyPixmapHeader we were asserting that a scratch pixmap's bo
does not have a dmabuf. We neglected to check whether the scratch
pixmap has a bo. If it doesn't this assert caused a segfault.

Change-Id: I9f2eee88f69e9d7c20a3506e618b3eadce716c2a

singulared/xf86-video-armsoc,xin3liang/xf86-video-armsoc,mmind/xf86-video-armsoc,OtherCrashOverride/armsoc-hardkernel,xin3liang/xf86-video-armsoc,freedesktop-unofficial-mirror/xorg__driver__xf86-video-armsoc,mmind/xf86-video-armsoc,freedesktop-unofficial-mirror/xorg__driver__xf86-video-armsoc,sbambrough/xf86-video-armsoc,singulared/xf86-video-armsoc,apxii/xf86-video-armsoc,apxii/xf86-video-armsoc,OtherCrashOverride/armsoc-hardkernel,sbambrough/xf86-video-armsoc