drm/i915: Fix potential bit_17 double-free

In the Linux kernel, the following vulnerability has been resolved:

drm/i915: Fix potential bit_17 double-free

A userspace with multiple threads racing I915_GEM_SET_TILING to set the

tiling to I915_TILING_NONE could trigger a double free of the bit_17

bitmask. (Or conversely leak memory on the transition to tiled.) Move

allocation/free'ing of the bitmask within the section protected by the

obj lock.

[tursulin: Correct fixes tag and added cc stable.]

(cherry picked from commit 10e0cbaaf1104f449d695c80bcacf930dcd3c42e)