gallium dri st: Probe the driver for supported surface formats.

gallium dri st: Probe the driver for supported surface formats.

This is done when constructing the fbconfigs, and the result is saved
for window system framebuffer creation.

Note: For dri2 the server needs to have an identical format selection
logic. Otherwise the dri state-tracker and the xorg driver (state-tracker)
will disagree on which format to use for the attachments. Some more work
is needed in this area.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>