avoid to create new textures for dri2

patch comes from radeon ddx. commit message was:
When creating a new pixmap/bo for dri2 drawable there is no need to create a
new texture, instead the texture associated with the new pixmap should be use
otherwise there is a missmatch between the bo backing the texture for the
drawable and the bo used by dri2 client.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>