main: Use a derived value for the default sample count

main: Use a derived value for the default sample count

Previously the framebuffer default sample count was taken directly
from the value given by the application. On the i965 driver on HSW if
the value wasn't one that is supported by the hardware it would hit an
assert when it tried to program the state for it. This patch fixes it
by adding a derived sample count to the state for the default
framebuffer. The driver can then quantize this to one of the valid
values in its UpdateState handler when the _NEW_BUFFERS state changes.
_mesa_geometric_samples is changed to use the new derived value.

Fixes the piglit test arb_framebuffer_no_attachments-query

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93957
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>