drm/i915: Avoid clearing the base drm_crtc_state

To prevent having to preserve the drm_crtc_state as we clear the
intel_crtc_state, only memset our extended state.

Fixes:
drivers/gpu/drm/i915/intel_display.c: In function ‘clear_intel_crtc_state’:
drivers/gpu/drm/i915/intel_display.c:11301:1: error: the frame size of 1056 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

v2: Add a comment and BUILD_BUG_ON to explain the memset()

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170303154644.6709-1-chris@chris-wilson.co.uk
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>