mesa: Set all viewports from _mesa_Viewport and _mesa_DepthRange

mesa: Set all viewports from _mesa_Viewport and _mesa_DepthRange

In _mesa_Viewport and _mesa_DepthRange, make sure that
ctx->Driver.Viewport is only called once instead of once per viewport or
depth range.

v2: Make _mesa_DepthRange actually set all of the depth ranges (instead
of just index 0).  Noticed by Ken.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>