svga: add/use new svga_sampler_format() function

svga: add/use new svga_sampler_format() function

This is important for the case of sampling from a depth texture.  In
that case, we need to sample the texture as if it were a single-channel
color texture.  For other/color formats, we can use the format as-is.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>