drm/i915/psr: Remove useless function calls.

PSR is no longer supported on VLV/CHV so this is just dead code.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180711220050.21809-1-rodrigo.vivi@intel.com