drm/i915: add relative ring register macros

Documentation explicitly mentions that the ring registers are
designed to have the same offsets relative to a base registers.

Use this to fight the code beaurocratic in intel_ringbuffer.c.

No code changes in this patch, just the new definitions.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index be1fd9b..3917d8b 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -20,6 +20,7 @@
 			u32 tail;
 			u32 start;
 	} regs;
+	u32		mmio_base;
 	unsigned long	size;
 	unsigned int	alignment;
 	void		*virtual_start;