[AGPGART] Rename a bunch of ambiguous variables.

mode = What X wants us to set the mode to (As set by AGPMode in X config)
cmd = PCI_AGP_STATUS from the AGP bridge.
tmp = PCI_AGP_STATUS from the graphic card.

mode -> requested_mode
cmd -> bridge_agpstat
tmp -> vga_agpstat
Signed-off-by: Dave Jones <davej@redhat.com>