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.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index f0b44d3..c1517b2 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -801,6 +801,7 @@
 		.tail = PRB0_TAIL,
 		.start = PRB0_START
 	},
+	.mmio_base		= RENDER_RING_BASE,
 	.size			= 32 * PAGE_SIZE,
 	.alignment		= PAGE_SIZE,
 	.virtual_start		= NULL,
@@ -839,6 +840,7 @@
 		.tail = BSD_RING_TAIL,
 		.start = BSD_RING_START
 	},
+	.mmio_base		= BSD_RING_BASE,
 	.size			= 32 * PAGE_SIZE,
 	.alignment		= PAGE_SIZE,
 	.virtual_start		= NULL,
@@ -957,6 +959,7 @@
                .tail   = GEN6_BSD_RING_TAIL,
                .start  = GEN6_BSD_RING_START
        },
+       .mmio_base		= GEN6_BSD_RING_BASE,
        .size			= 32 * PAGE_SIZE,
        .alignment		= PAGE_SIZE,
        .virtual_start		= NULL,