drm/i915: Don't grab crtc mutexes in intel_modeset_gem_init()

When the pipe A force quirk is applied the code will attempt to grab
a crtc mutex during intel_modeset_setup_hw_state(). If we're already
holding all crtc mutexes this will obviously deadlock every time.

So instead of using drm_modeset_lock_all() just grab the
mode_config.mutex. This is enough to avoid the unlocked mutex warnings
from certain lower level functions.

The regression was introduced in:

 commit 027476642811f8559cbe00ef6cc54db230e48a20
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Mon Dec 2 11:08:06 2013 +0200

    drm/i915: Take modeset locks around intel_modeset_setup_hw_state()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org
[danvet: Add cc: stable since the offending commit has that, too.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -11053,10 +11053,10 @@ void intel_modeset_gem_init(struct drm_device *dev)
 
 	intel_setup_overlay(dev);
 
-	drm_modeset_lock_all(dev);
+	mutex_lock(&dev->mode_config.mutex);
 	drm_mode_config_reset(dev);
 	intel_modeset_setup_hw_state(dev, false);
-	drm_modeset_unlock_all(dev);
+	mutex_unlock(&dev->mode_config.mutex);
 }
 
 void intel_modeset_cleanup(struct drm_device *dev)