drm/i915: Trim struct_mutex usage for kms

Reduce acquisition of struct_mutex to the critical regions that must
hold it; for KMS, we need struct_mutex currently only for the purpose of
pinning/unpinning the framebuffer's VMA into the global GTT. This allows
us to avoid taking the struct_mutex when disabling the CRTC (i.e. NULL
framebuffer objects) before a reset. (Not yet achieving the full goal of
avoiding the strut_mutex nesting, but good enough to break the first
half of the reset deadlock.)

v2: Keep pages pinning inside struct_mutex for the moment.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170726160038.29487-1-chris@chris-wilson.co.uk


[danvet: Drop another case of grabbing dev->struct_mutex around
cleanup_planes, which popped up because I had to redo the drm-next
backmerge for entirely different reasons. Acked by Chris on irc.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>