drm/i915: Only poll DW3_A when init DDI PHY for ports B and C.

According to Bspec we need to
"Poll for PORT_REF_DW3_A grc_done == 1b"
only on ports B and C initialization sequence when
copying rcomp from port A.

So let's follow the spec and only poll for that case
and not on every port A initialization.

v2: Also remove the grc_done check from bxt_ddi_phy_is_enabled()
    otherwise it might believe it is disabled and force it to re program.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479410256-25735-1-git-send-email-rodrigo.vivi@intel.com