drm/i915: make user mode sync polarity setting explicit

Userspace can pass a mode with an unspecified vsync/hsync polarity
setting. All encoders in the Intel driver take this to mean a negative
polarity setting. The HW readout/state checker code on the other hand
needs these flags to be explicitly set, otherwise the state checker will
WARN about the mismatch.

Get rid of the WARN by making the polarity setting explicit in the
adjusted mode flags based on the requested mode flags. This will keep
the existing behavior otherwise.

Note that we could guess from the other timing parameters whether the
user wanted a VESA or other standard mode and set the polarity
accordingly. This is what the NV driver does
(drivers/gpu/drm/nouveau/dispnv04/crtc.c), but I think that's not very
exact and would change the existing behavior of the Intel driver.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65442


Signed-off-by: Imre Deak <imre.deak@intel.com>
Tested-by: cancan,feng <cancan.feng@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>