drm/rcar: gem: dumb: pitch is an output

When creating a dumb buffer object using the DRM_IOCTL_MODE_CREATE_DUMB
IOCTL, only the width, height, bpp and flags fields are inputs. The
caller is not guaranteed to zero out or set handle, pitch and size.
Drivers must not treat these values as possible inputs, otherwise they
may use uninitialized memory during the computation of the framebuffer
size.

The R-Car DU driver treats the pitch passed in from userspace as minimum
and will only overwrite it when the driver-computed pitch is larger,
allowing userspace to, intentionally or not, overallocate framebuffers.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>