[ARM] pxafb: allow pxafb_set_par() to start from arbitrary yoffset

Note the var->yres_virtual is only re-calculated from the fix.smem_len
when text mode acceleration is enabled (which is default), this is due
to the issue as Russell suggested below:

Previous experience of doing this with the X server and acornfb is that
it causes all sorts of problems - it seems to force the X server into
assuming that the framebuffer should be panned no matter what settings
you ask it for.

The recommended workaround (implemented in acornfb) is to only do these
kinds of adjustments if text mode acceleration is enabled.  IIRC, the X
server should be disabling text mode acceleration when it maps the
framebuffer.  I seem to remember that there are X servers which forget
to do that though.

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Eric Miao <ycmiao@ycmiao-hp520.(none)>