x86/boot: make "vga=current" work with graphics modes

x86/boot: make "vga=current" work with graphics modes

GrUB2 can be told to leave the screen in the graphics mode it has been
using (or any other one), via "set gfxpayload=keep" (or suitable
variants thereof). In this case we can avoid doing another mode switch
ourselves. This in particular avoids possibly setting the screen to a
less desirable mode: On one of my test systems the set of modes
reported available by the VESA BIOS depends on whether the interposed
KVM switch has that machine set as the active one. If it's not active,
only modes up to 1024x768 get reported, while when active 1280x1024
modes are also included. For things to always work with an explicitly
specified mode (via the "vga=" option), that mode therefore needs be a
1024x768 one.

For some reason this only works for me with "multiboot2" (and
"module2"); "multiboot" (and "module") still forces the screen into text
mode, despite my reading of the sources suggesting otherwise.

For starters I'm limiting this to graphics modes; I do think this ought
to also work for text modes, but
- I can't tell whether GrUB2 can set any text mode other than 80x25
  (I've only found plain "text" to be valid as a "gfxpayload" setting),
- I'm uncertain whether supporting that is worth it, since I'm uncertain
  how many people would be running their systems/screens in text mode,
- I'd like to limit the amount of code added to the realmode trampoline.

For starters I'm also limiting mode information retrieval to raw BIOS
accesses. This will allow things to work (in principle) also with other
boot environments where a graphics mode can be left in place. The
downside is that this then still is dependent upon switching back to
real mode, so retrieving the needed information from multiboot info is
likely going to be desirable down the road.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Henry Wang <Henry.Wang@arm.com>