Add a small helper to parse the PSR setup time from the DPCD PSR
capabilities and return the value in microseconds.

v2: Don't waste so many bytes on the psr_setup_time_us[] table

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>