drm/i915: struct_mutex is not required for allocating the framebuffer

We do not need the BKL struct_mutex in order to allocate a GEM object,
nor to create the framebuffer, so resist the temptation to take the BKL
willy nilly. As this changes the locking contract around internal API
calls, the patch is a little larger than a plain removal of a pair of
mutex_lock/unlock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215105919.7347-2-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>