drm/i915/vlv: WA to fix Voltage not getting dropped to Vmin when Gfx is power gated.

When we enter RC6 and GFX Clocks are off, the voltage remains higher
than Vmin. When we try to set the freq to RPn, it might fail since the
Gfx clocks are down. So to fix this in Gfx idle, Bring the GFX clock up
and set the freq to RPn then move GFx down.

v2: remove vlv_update_rps_cur_delay function. Update commit message (Daniel)

v3: Fix the timeout during wait for gfx clock (Jesse)

v4: addressed comments on set freq and punit wait (Ville)

v5: use wait_for while waiting for GFX clk to be up. (Daniel)
    update cur_delay before requesting min_delay. (Ville)

v6: use wait_for while waiting for punit. (Ville)

Signed-off-by: Deepak S <deepak.s@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>