drm/i915/dsc: Compute Rate Control parameters for DSC

This computation of RC params happens in the atomic commit phase
during compute_config() to validate if display stream compression
can be enabled for the requested mode.

v7 (From Manasi):
* Use DRM_DEBUG instead of DRM_ERROR (Ville)
* Use Error numberinstead of -1 (Ville)
v6 (From Manasi):
* Use 9 instead of 0x9 for consistency (Anusha)

v5 (From Manasi):
* Fix dim checkpatch warnings/checks
v4(From Gaurav):
* No change.Rebase on drm-tip

v3 (From Gaurav):
* Rebase on top of Manasi's latest series
* Return -ve value in case of failure scenarios (Manasi)

Fix review comments from Ville:
* Remove unnecessary comments
* Remove unnecessary paranthesis
* Add comments for few RC params calculations

v2 (From Manasi):
* Rebase Gaurav's patch from intel-gfx to gfx-internal
* Use struct drm_dsc_cfg instead of struct intel_dp
as a parameter

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Signed-off-by: Gaurav K Singh <gaurav.k.singh@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181128202628.20238-5-manasi.d.navare@intel.com