svga: Condition preemptive flush on draw emission

svga: Condition preemptive flush on draw emission

On ultra high resolution modes, the preemptive flush flag can be
set midway through command submission, a condition that cannot be
recovered from a flush-retry, causing rendering artifacts.

This patch prevents a preemtive_flush until a draw has been
emitted.

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>