drm/i915: Add intel_atomic_get_existing_crtc_state function

This patch Adds a function to extract intel_crtc_state from the
atomic_state, if not available it returns NULL.

v2 (from Paulo):
 - Fix white space problem detected by checkpatch.

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161201154940.24446-7-mahesh1.kumar@intel.com