drm/i915/dp: Add helpers for Compressed BPP and Slice Count for DSC

This patch adds helpers for calculating the maximum compressed BPP
supported with small joiner.
This also adds a helper for calculating the slice count in case
of small joiner.
These are inside intel_dp since they take into account hardware
limitations.

v6:
* Take mode_clock and mode_hdisplay as input arguments
so that this can be called in intel_dp_mode_valid (Manasi)
v5:
* Get the max slice width from DPCD
* Check against Min_Slice_width of 2560 (Anusha)
v4:
* #defines for PPR in slice count helper (Gaurav)
v3:
* Simply logic for bpp (DK)
* Limit the valid slice count by max supported by Sink (Manasi)
v2:
* Change the small joiner RAM buffer constant as bspec changed (Manasi)
* rename it as SMALL_JOINER since we are not enabling
big joiner yet (Anusha)

Cc: Gaurav K Singh <gaurav.k.singh@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Reviewed-by: Gaurav K Singh <gaurav.k.singh@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181031001923.31442-5-manasi.d.navare@intel.com