x86/setup: Move trim_snb_memory() later in setup_arch() to fix boot hangs

Commit

  a799c2bd ("x86/setup: Consolidate early memory reservations")

moved reservation of the memory inaccessible by Sandy Bride integrated
graphics very early, and, as a result, on systems with such devices
the first 1M was reserved by trim_snb_memory() which prevented the
allocation of the real mode trampoline and made the boot hang very
early.

Since the purpose of trim_snb_memory() is to prevent problematic pages
ever reaching the graphics device, it is safe to reserve these pages
after memblock allocations are possible.

Move trim_snb_memory() later in boot so that it will be called after
reserve_real_mode() and make comments describing trim_snb_memory()
operation more elaborate.

 [ bp: Massage a bit. ]

Fixes: a799c2bd ("x86/setup: Consolidate early memory reservations")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Hugh Dickins <hughd@google.com>
Link: https://lkml.kernel.org/r/f67d3e03-af90-f790-baf4-8d412fe055af@infradead.org