vgabios: Make struct vgamode_s more similar to bochs/cirrus mode tables.

vgabios: Make struct vgamode_s more similar to bochs/cirrus mode tables.

For graphics modes, store pixel width/height instead of text
width/height.  Add explicit char width field.  Where needed, calculate
text width/height from pixel width/height by dividing pixel count by
character size.

Rename some fields and change field sizes to match cirrus/bochs
definitions.

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