drm/i915/crt: Do not rely upon the HPD presence pin

drm/i915/crt: Do not rely upon the HPD presence pin

VGA hotplug detection "works" by measuring the resistance across
certain pins. A lot of kvm switches fumble this and wire up cheap
resistors with the wrong resistance or don't bother at all.

To accomodate these, also try to detect a connected monitor by trying
to grab the edid. Contrary to !HAS_HOTPLUG platforms we don't bother
with an actual load-detection cycle when the output is life - that
would be actual work to implement because things moved around. This is
the big difference to Chris Wilson's original approach:

commit 9e612a008fa7fe493a473454def56aa321479495
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu May 31 13:08:53 2012 +0100

    drm/i915/crt: Do not rely upon the HPD presence pin

This blew up on Linus' machine because it errornously detected a vga
screen (without and edid and hence only the default modes), leading to
it's prompt removal:

commit 8f53369b753f5f4c7684c2eb0b592152abb1dd00
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri Jun 8 14:53:06 2012 -0700

    Revert "drm/i915/crt: Do not rely upon the HPD presence pin"

Some digging around in Bspec shows the reason why load detect doesn't work on
newer chips - the legacy VGA load detect bit isn't wired up any longer:

Public Snb Bspec, Vol3 Part1, 1.1.1 ST00 Input Status 0, bit4:

"RGB Comparator / Sense. This bit is here for compatibility and will
always return one. Monitor detection must be done be done through the
programming of registers in the MMIO space.
0 = Below threshold
1 = Above threshold"

v2: Add a comment in the code that load detect on hotplug capable
machines is broken and pimp the commit message with a quote of Bspec
to show why.

Reported-and-tested-by: Matthieu LAVIE <boiteamadmax@hotmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50501
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>