drm/i915: Push clear_intel_crtc_state() onto the heap

clear_intel_crtc_state() uses the stack for saving a temporary copy of
certain bits of the inherited crtc_state before clearing the unwanted
bits. This pushes it over the stack limit for my little 32b Pineview,
so move the temporary allocation to the heap instead. As we now use a
zeroed struct, we can copy the whole extended state back to both
preserve what bits need to be preserved and zero the rest.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190205092759.16018-1-chris@chris-wilson.co.uk