mesa/st: Use _mesa_geometric_ functions appropriately

mesa/st: Use _mesa_geometric_ functions appropriately

Change references to gl_framebuffer::Width, Height, MaxNumLayers
and Visual::samples to use the _mesa_geometric_ convenience functions
for those places where the geometry of the gl_framebuffer is needed.
This is in contrast to the geometry of the intersection of the
attachments of the gl_framebuffer.

This patch paves the way to enable GL_ARB_framebuffer_no_attachements
for all gallium drivers.

V.2:
 Remove itermeditate variable state.

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>