drm/i915: Share the computation of ring size for RING_CTL register

Since both legacy and execlists want to populate the RING_CTL register,
share the computation of the right bits for the ring->size. We can then
stop masking errors and explicitly forbid them during creation!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161004201132.21801-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_lrc.c

@@ -1946,7 +1946,7 @@ populate_lr_context(struct i915_gem_context *ctx,
 		       RING_START(engine->mmio_base), 0);
 	ASSIGN_CTX_REG(reg_state, CTX_RING_BUFFER_CONTROL,
 		       RING_CTL(engine->mmio_base),
-		       ((ring->size - PAGE_SIZE) & RING_NR_PAGES) | RING_VALID);
+		       RING_CTL_SIZE(ring->size) | RING_VALID);
 	ASSIGN_CTX_REG(reg_state, CTX_BB_HEAD_U,
 		       RING_BBADDR_UDW(engine->mmio_base), 0);
 	ASSIGN_CTX_REG(reg_state, CTX_BB_HEAD_L,