drm/i915/icl: Add DSS_CTL Registers

Add defines for DSS_CTL registers.
These registers specify the big joiner, splitter,
overlap pixels and info regarding
compression enabled on left or right branch.

v2:
- rebase. Remove overlapping defines(James Ausmus)
- Rename the register to ICL_DSS_CTL1/2_PIPE_ (manasi)
- take pixels as an argument for overlap.(Manasi)

v3:
- rebase. merge DSS_CTL1/2 introduced in Madhav's patch
  to avoid confusion (madhav chauhan)
- Rename registers in accordance to BSpec (Madhav, Rodrigo)
- Add define to conditionally check the buffer target depth (James Ausmus)

v4:
- remove redundant definitions.(madhav)

v5:
- Add mask for overlap pixels.
- Code Style changes.(Madhav)
v6:
- Code style changes. (Madhav)

Suggested-by: Madhav Chauhan <madhav.chauhan@intel.com>
Cc: Madhav Chauhan <madhav.chauhan@intel.com>
cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: James Ausmus <james.ausmus@intel.com>
Cc: Gaurav Singh <gaurav.k.singh@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@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/07021336cb87d09e8f97fbff709c4e686d7de536.1540900289.git.jani.nikula@intel.com