drm/i915: don't send DP "idle" pattern before "normal" on HSW PORT_A

The DP_TP_STATUS register for PORT_A doesn't exist. Our documentation
will be fixed soon, so the code does not match it for now.

This solves "Timed out waiting for DP idle patterns" and "unclaimed
register" messages on eDP.

V1: Was called "drm/i915: don't read DP_TP_STATUS(PORT_A)"
V2: Was called "drm/i915: don't send DP idle pattern before normal
pattern on HSW"
V3: Only change the code that touches PORT_A.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>