drm/i915: Fix NULL ptr deref by checking new_crtc_state

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

drm/i915: Fix NULL ptr deref by checking new_crtc_state

intel_atomic_get_new_crtc_state can return NULL, unless crtc state wasn't

obtained previously with intel_atomic_get_crtc_state, so we must check it

for NULLness here, just as in many other places, where we can't guarantee

that intel_atomic_get_crtc_state was called.

We are currently getting NULL ptr deref because of that, so this fix was

confirmed to help.

(cherry picked from commit 1d5b09f8daf859247a1ea65b0d732a24d88980d8)