sna: CustomEDID fix

For my HTPC setup, I'm using the option "CustomEDID".
With this option, output attaching and destroying events leads to
crashes.

The following sequence leads to a crash:
- In xorg.conf: Option "CustomEDID" "HDMI2:/etc/my_edid.bin"
- Starting Xorg
- Connect HDMI2
- Disconnect HDMI2
- Reconnect HDMI2
  -> Crash

The crash happens in xf86OutputSetEDID
(xorg/xserver/hw/xfree86/modes/xf86Crtc.c)
at "free(output->MonInfo)". MonInfo is assigned with
sna_output->fake_edid_mon
which is allocated by intel driver in sna_output_load_fake_edid
(src/sna/sna_display.c).

Sequence details:
- Starting Xorg
   -> fake_edid_mon is initialized

- Connect HDMI2
   -> xf86OutputSetEDID is called:
       - MonInfo is NULL
       - MonInfo is assigned with fake_edid_mon pointer
       - MonInfo is read by Xorg

- Disconnect HDMI2

- Reconnect HDMI2
   -> xf86OutputSetEDID is called:
       - MonInfo is freed thus also fake_edid_mon
       - MonInfo is assigned with fake_edid_mon
       - MonInfo is read but it was freed -> CRASH

The fix consists of a new instance of xf86MonPtr for each calls of
xf86OutputSetEDID.
is initialized with fake_edid_raw which render
fake_edid_mon useless.
With this proposal, the behaviour of an EDID override is similar to
a "real" EDID.

Signed-off-by: Dominique Constant <dom.constant@free.fr>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>