drm/i915: Try harder to get best m, n, p values with minimal error

Basically check for both +ive and -ive deviation from target clock and
pick the one with minimal error. If we get a direct match, break from
loop to acheive some optimization.

v2: Use signed variable for target and calculated dsi clock values

Signed-off-by: Vijayakumar Balakrishnan <vijayakumar.balakrishnan@intel.com>
Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>