drm/i915/dsi: Add vblank calculation for command mode

Transcoder timing calculation differ for command mode.

v2: Use is_vid_mode, and use same I915_WRITE (Jani)
v3: Adjust the calculations to reflect dsc compression ratio
v4: Rearrange the vertical and horizontal timing calc, optimize
    local variables usage. (Jani)
v5: Fix the values used for calculation, use afe_clk for
    byte clock calculation, use intel_de_write/read (Jani)

Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200312053841.2794-3-vandita.kulkarni@intel.com