drm/i915: set the LPT FDI RX polarity reversal bit when needed

If we fail to set the bit when needed we get some nice FDI link
training failures (AKA "black screen on VGA output").

While we don't really know how to properly choose whether we need to
set the bit or not (VBT?), just read the initial value set by the BIOS
and store it for later usage.

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

drivers/gpu/drm/i915/i915_drv.h

@@ -915,6 +915,8 @@ typedef struct drm_i915_private {
 	bool hw_contexts_disabled;
 	uint32_t hw_context_size;
 
+	bool fdi_rx_polarity_reversed;
+
 	struct i915_suspend_saved_registers regfile;
 
 	/* Old dri1 support infrastructure, beware the dragons ya fools entering