drm/i915/psr: remove drmP.h include that crept in

We've been free of deprecated drmP.h includes for a while, but one crept
in. Fend it off.

Cc: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190318160409.27648-1-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_psr.c

@@ -51,7 +51,6 @@
  * must be correctly synchronized/cancelled when shutting down the pipe."
  */
 
-#include <drm/drmP.h>
 #include <drm/drm_atomic_helper.h>
 
 #include "intel_drv.h"