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