drm/atomic: Handling the case when setting old crtc for plane

In the func drm_atomic_set_crtc_for_plane, with the current code,
if crtc of the plane_state and crtc passed as argument to the func
are same, entire func will executed in vein.
It will get state of crtc and clear and set the bits in plane_mask.
All these steps are not required for same old crtc.
Ideally, we should do nothing in this case, this patch handles the same,
and causes the program to return without doing anything in such scenario.

Signed-off-by: Satendra Singh Thakur <satendra.t@samsung.com>
Cc: Madhur Verma <madhur.verma@samsung.com>
Cc: Hemanshu Srivastava <hemanshu.s@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1525326572-25854-1-git-send-email-satendra.t@samsung.com