video: s3c-fb: fix virtual resolution checking

This patch fixes mishandling in virtual resolution checking.
Previously, virtual resolution is changed to virtual_x and virtual_y
which mean the size for buffer allocation, when s3c_fb_check_var is
called by fb_check_var. However, it is meaningless, since virtual_x
and virtual_y are fixed and user cannot change virtual resolution.
Therefore, virtual resolution should be more than resolution
such as xres and yres.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>