drm/udl: fix stride issues scanning out stride != width*bpp

When buffer sharing with the i915 and using a 1680x1050 monitor,
the i915 gives is a 6912 buffer for the 6720 width, the code doesn't
render this properly as it uses one value to set the base address for
reading from the vmap and for where to start on the device.

This fixes it by calculating the values correctly for the device and
for the pixmap. No idea how I haven't seen this before now.

Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>