vgabios: Unify page size calculations; remove page size from vgamode_s.

vgabios: Unify page size calculations; remove page size from vgamode_s.

The previous code could obtain the page size in different ways - from
vmode_g->sslength, from SCREEN_MEM_START, or by manually multiplying
cols and rows.  Add a new func (calc_page_size) and use that in areas
that need to calculate the page size.

Convert readers of the page size to read it from the "video_pagesize"
entry in the BDA instead of recalculating it.

Remove the page size from struct vgamode_s (slength) as it is now
calculated dynamically.  The new calculated versions are different
from the existing values exported in video_param_table.  However, the
existing values (for CGA) did not look correct - I compared the values
to those exported by two other VGA BIOS manufacturers and used the
values that look more standard.

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