drm/i915/bxt: Adjusting the error in horizontal timings retrieval

In BXT DSI there is no regs programmed with few horizontal timings
in Pixels but txbyteclkhs.. So retrieval process adds some
ROUND_UP ERRORS in the process of PIXELS<==>txbyteclkhs.

Actually here for the given adjusted_mode, we are calculating the
value programmed to the port and then back to the horizontal timing
param in pixels. This is the expected value at the end of get_config,
including roundup errors. And if that is same as retrieved value
from port, then retrieved (HW state) adjusted_mode's horizontal
timings are corrected to match with SW state to nullify the errors.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461053894-5058-2-git-send-email-ramalingam.c@intel.com
(cherry picked from commit 042ab0c3

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>