drm/tilcdc: Remove drm_framebuffer_get() and *_put() calls

The drm_framebuffer_get() and drm_framebuffer_put() calls in the
tilcdc driver are obsolete. The drm atomic modesetting core should
take care of holding the references while the atomic state object is
in use. The old state is deleted when a commit of a new one is
completed after drm_atomic_helper_wait_for_vblanks().

This also fixes an occasional framebuffer leak the old
drm_framebuffer_get() and drm_framebuffer_put() code had.

Signed-off-by: Jyri Sarha <jsarha@ti.com>