svga: fix missing format multisample devcap check

In commit e4048f6cd1, svga_is_dx_format_supported() is supposed to
also check the SVGA3D_DXFMT_MULTISAMPLE bit for multisample
support of a format. Somehow that code is not included in that commit.
This patch fixes it.

Fixes piglit test spec@ext_framebuffer_multisample@formats all_samples.

Reviewed-by: Brian Paul <brianp@vmware.com>