drm/komeda: Add drm_ctm_to_coeffs()

This function is for converting drm_color_ctm matrix to komeda hardware
required required Q2.12 2's complement CSC matrix.

v2:
  Move the fixpoint conversion function s31_32_to_q2_12() to drm core
  as a shared helper.

Signed-off-by: james qian wang (Arm Technology China) <james.qian.wang@arm.com>
Reviewed-by: Mihail Atanassov <mihail.atanassov@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191112110927.20931-4-james.qian.wang@arm.com