drm/vmwgfx: Fix fbdev emulation using legacy functions

drm/vmwgfx: Fix fbdev emulation using legacy functions

I've broken this by removing the backoff handling from the
set_config2atomic helper in

commit 38b6441e4e75c0b319cfe4d9364c1059fc1e3c2b
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Mar 22 22:50:58 2017 +0100

    drm/atomic-helper: Remove the backoff hack from set_config

Fixing this properly would mean we get to wire the acquire_ctx all the
way through vmwgfx fbdev code, and doing the same was tricky for the
shared fbdev layer. Probably much better to look into refactoring the
entire code to use the helpers, but since that's not a viable
long-term solution fix the issue by open-coding a vmwgfx version of
set_config, that does the legacy backoff dance internally.

Note: Just compile-tested. The idea is to take
drm_mode_set_config_internal(), remove the "is this a legacy driver"
check, and whack the drm_atomic_legacy_backoff trickery at the end.
Since drm_atomic_legacy_backoff is for atomic commits only we need to
open-code it.

Cc: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170406200256.26040-1-daniel.vetter@ffwll.ch