Add VESA mode support for syscons. The vesa.ko module must be loaded and

Add VESA mode support for syscons.  The vesa.ko module must be loaded and
syscons (sc0) must be compiled with 'options SC_PIXEL_MODE'.
'vidcontrol -i mode' may then be used to list available modes and
'vidcontrol MODE_n' may be used to set the mode.  This patch adds support
for 15, 16, 24, and 32 bit modes and also does a bit of related cleanup and
restructuring.

Submitted-by: Sascha Wildner <saw@online.de>
Testing-and-Discussion: Sarunas Vancevicius, YONETANI Tomokazu and Joerg Anslik