drm/i915: Convert wait_for(I915_READ(reg)) to intel_wait_for_register()

By using the out-of-line intel_wait_for_register() not only do we can
efficiency from using the hybrid wait_for() contained within, but we
avoid code bloat from the numerous inlined loops, in total (all patches):

   text    data     bss     dec     hex filename
1078551    4557     416 1083524  108884 drivers/gpu/drm/i915/i915.ko
1070775    4557     416 1075748  106a24 drivers/gpu/drm/i915/i915.ko
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467297225-21379-58-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_sideband.c

@@ -240,7 +240,8 @@ void intel_sbi_write(struct drm_i915_private *dev_priv, u16 reg, u32 value,
 
 	WARN_ON(!mutex_is_locked(&dev_priv->sb_lock));
 
-	if (wait_for((I915_READ(SBI_CTL_STAT) & SBI_BUSY) == 0,
+	if (intel_wait_for_register(dev_priv,
+				    SBI_CTL_STAT, SBI_BUSY, 0,
 				    100)) {
 		DRM_ERROR("timeout waiting for SBI to become ready\n");
 		return;