drm/i915/icl: Fix DDI/TC port clk_off bits

drm/i915/icl: Fix DDI/TC port clk_off bits

DDI/TC clock-off bits are not equally distanced. TC1-3 bits are
from offset 12 & TC4 is at offset 21.
Create a function to choose correct clk-off bit.

v2: Add fixes tag (Lucas)

Fixes: c27e917e2bda ("drm/i915/icl: add basic support for the ICL clocks")
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181016023752.9285-1-lucas.demarchi@intel.com