drm/i915: Use the fixed pixel clock for eDP in intel_dp_set_m_n()

drm/i915: Use the fixed pixel clock for eDP in intel_dp_set_m_n()

commit 9d1a455b0ca1c2c956b4d9ab212864a8695270f1 upstream.

The eDP output on HP Z1 is still broken when X is started even after
fixing the infinite link-train loop.  The regression was introduced in
3.6 kernel for cleaning up the mode clock handling code in intel_dp.c
by the commit [71244653: drm/i915: adjusted_mode->clock in the dp
mode_fix].

In the past, the clock of the reference mode was modified in
intel_dp_mode_fixup() in the case of eDP fixed clock, and this clock was
used for calculating in intel_dp_set_m_n().  This override was removed,
thus the wrong mode clock is used for the calculation, resulting in a
psychedelic smoking output in the end.

This patch corrects the clock to be used in the place.

v1->v2: Use intel_edp_target_clock() for checking eDP fixed clock
instead of open code as in ironlake_set_m_n().

Backported for 3.8-stable. Reverted refactoring in e69d0bc1.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Lingzhu Xiang <lxiang@redhat.com>
Reviewed-by: CAI Qian <caiqian@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>