drm/i915: kill superflous IS_I855 macro

It is identical to I85X. Use that one instead.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[anholt: fix conflicts against the display function pointer stuff]
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_drv.h

@@ -946,7 +946,6 @@ extern int i915_wait_ring(struct drm_device * dev, int n, const char *caller);
 #define IS_I830(dev) ((dev)->pci_device == 0x3577)
 #define IS_845G(dev) ((dev)->pci_device == 0x2562)
 #define IS_I85X(dev) ((dev)->pci_device == 0x3582)
-#define IS_I855(dev) ((dev)->pci_device == 0x3582)
 #define IS_I865G(dev) ((dev)->pci_device == 0x2572)
 
 #define IS_I915G(dev) ((dev)->pci_device == 0x2582 || (dev)->pci_device == 0x258a)