svga: re-validate sampler view at draw time if needed

svga: re-validate sampler view at draw time if needed

This patch validates those sampler views with backing copy
of texture whose original copy has been updated since the
view is last validated.
This is done here at draw time because the texture binding might not
have modified, hence validation is not triggered at state update time,
and yet the texture might have been updated in another context, so
we need to re-validate the sampler view in order to update the backing
copy of the updated texture.

This fixes a rendering flickering issue with Photoshop running in
Linux VM with HWversion 11. The problem is Photoshop renders to texture A
in context X, and then bind texture A to context Y. The first time
when texture A is bound to context Y, cso calls pipe->set_sampler_views().
Validation of sampler views is done, rendering is fine.
But when texture A is rendered to again in context X, and rebound in
context Y, cso skips pipe->set_sampler_views() because texture A is already
bound in context Y. SVGA driver is not given a chance to re-validate
the texture binding, the backing copy of the texture is not updated,
and hence causes black image.

Tested with Photoshop, MTT glretrace, piglit.
Fixes VMware bug 1769103.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>