gallium: add a texture target to sampler view and a CAP to use it

gallium: add a texture target to sampler view and a CAP to use it

This allows a sampler view to have a different texture target than the
underlying resource. This will be used to implement the type casting
between 2d arrays and cube maps as specified in ARB_texture_view.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>