drm/i915: implement dma buf begin_cpu_access (v2)

In order for udl vmap to work properly, we need to push the object
into the CPU domain before we start copying the data to the USB device.

This along with the udl change avoids userspace explicit mapping to
be used.

v2: add a flag for userspace to query to know if Intel kernel driver can
deal with the vmap flushing properly. In theory udl would need a flag also,
but I intend to push the patches very close to each other and other drivers
should do the right thing from the start.

I've added a test to my intel-gpu-tools prime branch, however testing
this is a bit messy since the only way to get udl to vmap is to rendering
something. I've tested this with real code as well to make sure it works.

Signed-off-by: Dave Airlie <airlied@redhat.com>
[danvet: resolved conflict, which required reallocating the PARAM
number to 21.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/i915_drm.h

@@ -312,6 +312,7 @@ typedef struct drm_i915_irq_wait {
 #define I915_PARAM_HAS_ALIASING_PPGTT	 18
 #define I915_PARAM_HAS_WAIT_TIMEOUT	 19
 #define I915_PARAM_HAS_SEMAPHORES	 20
+#define I915_PARAM_HAS_PRIME_VMAP_FLUSH	 21
 
 typedef struct drm_i915_getparam {
 	int param;