vgasrc: Use curmode_g instead of vmode_g when mode is the current video mode

vgasrc: Use curmode_g instead of vmode_g when mode is the current video mode

Many functions are passed a pointer to the current video mode
vgamode_s struct.  Use the name 'curmode_g' for these functions and
use 'vmode_g' for functions that can accept an arbitrary video mode.
Hopefully this will make the goals of the functions more clear.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>