i965: Use _mesa_geometric_ functions appropriately

i965: Use _mesa_geometric_ functions appropriately

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

This patch is to pave the way to enable GL_ARB_framebuffer_no_attachments
on Gen7 and higher in i965.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Kevin Rogovin <kevin.rogovin@intel.com>