intel: Don't _mesa_problem when asked for an RB of a texturing-only type.

intel: Don't _mesa_problem when asked for an RB of a texturing-only type.

We want to be able to support some formats for texturing that we can't
render to, which means that some choices for RenderbufferStorage end
up being incomplete (for example, L8 currently).  For these, where we
don't render to them, we don't want to have to make up an rb->DataType
that's only used for GetRow()/PutRow().