gma500: remove the legacy PM method

PCI core only prefer one of legacy PM and new PM. And since runtime pm
is implemented, which requires the new PM method, we should remove the
legacy PM method.

Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>