drm/i915: Consolidate checks for memcpy-from-wc support

In order to silence sparse:

../drivers/gpu/drm/i915/i915_gpu_error.c:200:39: warning: Using plain integer as NULL pointer

add a helper to check whether we have sse4.1 and that the desired
alignment is valid for acceleration.

v2: Explain the macros and split the two use cases between
i915_has_memcpy_from_wc() and i915_can_memcpy_from_wc().

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170106152013.24684-1-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/i915_cmd_parser.c b/drivers/gpu/drm/i915/i915_cmd_parser.c
index 6c310a8..21b1cd9 100644
--- a/drivers/gpu/drm/i915/i915_cmd_parser.c
+++ b/drivers/gpu/drm/i915/i915_cmd_parser.c
@@ -1079,7 +1079,7 @@ static u32 *copy_batch(struct drm_i915_gem_object *dst_obj,
 
 	src = ERR_PTR(-ENODEV);
 	if (src_needs_clflush &&
-	    i915_memcpy_from_wc((void *)(uintptr_t)batch_start_offset, NULL, 0)) {
+	    i915_can_memcpy_from_wc(NULL, batch_start_offset, 0)) {
 		src = i915_gem_object_pin_map(src_obj, I915_MAP_WC);
 		if (!IS_ERR(src)) {
 			i915_memcpy_from_wc(dst,