dri2: Flush in dri2_create_buffer2 after calling glamor_set_pixmap_bo

To make sure the client can't use the shared pixmap storage for direct
rendering first, which could produce garbage.

Bugzilla: https://bugs.freedesktop.org/109235
(Ported from amdgpu commit ebd32b1c07208f8dbe853e089f5e4b7c6a7a658a)