drm/i915: Add rotation support for cursor plane (v5)

The cursor plane also supports 180 degree rotation. Add a new
"cursor-rotation" property on the crtc which controls this.

Unlike sprites, the cursor has a fixed size, so if you have a small
cursor image with the rest of the bo filled by transparent pixels,
simply flipping the rotation property will cause the visible part
of the cursor to shift. This is something to keep in mind when
using cursor rotation.

v2: Fix gen4/vlv by offsetting the base address appropriately

v3: Removing cursor-rotation property and using rotation property on cursor
plane.
v4: Changing the author name back to Ville.

v5 (by Matt Roper): Slight tweaking to apply against latest di-nightly
codebase.

Cc: Sagar Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by (IVB): Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>