drm: xilinx: fb: Support the virtual resolution

drm: xilinx: fb: Support the virtual resolution

A framebuffer with virtual height can be allocated with this patch.
This would allow fbdev application to use that large framebuffer
as multiple buffers and pan display. The multiplier can be specified
by a module parameter, xilinx_drm.fbdev_vres, and the default value
is '2' to enable double-buffering as default.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>