drm/rcar-du: Fix maximum frame buffer pitch computation

drm/rcar-du: Fix maximum frame buffer pitch computation

The maximum pitch constraint for the hardware is expressed in pixels.
Convert it to bytes to validate frame buffer creation, as frame buffer
pitches are expressed in bytes.

Reported-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 8bed5cc765ffdd61b59f8405d38b377f5a7f0920)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>