drm/i915: BXT DDI PHY sequence BUN

According to the BSpec update, bit 7 of PORT_CL1CM_DW0 register needs to be
checked to ensure that the register is in accessible state.
Also, based on a BSpec update, changing the timeout value to
check iphypwrgood, from 10ms to wait for up to 100us.

v2: [Ville] use wait_for_us instead of the atomic call.
v3: [Jani/Imre] read register only once

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Reported-by: Philippe Lecluse <Philippe.Lecluse@intel.com>
Cc: Deak, Imre <imre.deak@intel.com>
Cc: Nikula, Jani <jani.nikula@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459446354-19012-1-git-send-email-vandana.kannan@intel.com