svga: don't call svga_texture_device_format_has_alpha() for PIPE_BUFFER

svga: don't call svga_texture_device_format_has_alpha() for PIPE_BUFFER

svga_texture_device_format_has_alpha() is only intended to work for
texture resources, not buffer resources.  This fixes a failed assertion
in the svga_texture() cast function when running texture buffer tests.

Also, add an assertion in svga_texture_device_format_has_alpha() to
catch the issue sooner.

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