drm/i915: Fix pipe config warnings when dealing with LVDS fixed mode

intel_fixed_panel_mode() overwrote the adjusted_mode with the fixed mode
only partially. Notably it forgot to copy over the sync flags. The LVDS code however programmed the hardware with the sync flags from fixed mode, and then later the pipe config comparison obviously failed as we
filled out the adjusted_mode in get_config from the real registers.

Just call drm_mode_copy() in intel_fixed_panel_mode() to copy over the
whole thing, and then just use adjusted_mode in the LVDS code to figure
out which sync settings the hardware needs.

Also constify the fixed_mode argument to intel_fixed_panel_mode().

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