drm/i915: Remove repeated declaration

struct drm_i915_private, struct intel_crtc_state and
struct intel_crtc is declared twice.
Remove the duplicate.

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210326012527.875026-1-wanjiabing@vivo.com