drm/i915/icl: Define register for DSI PLL

This patch adds the new registers and corresponding bit definitions
which will be used for programming/enable DSI PLL.

v2: Review comments from Jani N
    - Fix spaces while defining ICL_ESC_CLK_DIV_MASK
    - Define shift and mask for bitfields.

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1530795727-28644-2-git-send-email-madhav.chauhan@intel.com