drm/i915: Use adjusted_mode appropriately when computing watermarks

Currently most of the watermark code looks at crtc->mode which is the
user requested mode. The only piece of information there that is
relevant is hdisplay, the rest must come from adjusted_mode. Convert
all of the code to use requested_mode and adjusted_mode from
pipe config appropriately.

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