Force framebuffer virtual viewport to be the same as physical

VideoCore reports garbage in viewport geometry fields unless
viewport was set previously by earlier stage boot loader. So
when booting FreeBSD kernel directly from VideoCore's start.elf
framebuffer intialization fails due to invalid vxres, vyres
values. Make sure we request viewport to be equal to physical
resolution

Submitted by: Sylvain Garrigues <sylvain@sylvaingarrigues.com>