drm/i915: Introduce parameterized DBUF_CTL

Now start using parameterized DBUF_CTL instead
of hardcoded, this would allow shorter access
functions when reading or storing entire state.

Tried to implement it in a MMIO_PIPE manner, however
DBUF_CTL1 address is higher than DBUF_CTL2, which
implies that we have to now subtract from base
rather than add.

v2: - Removed unneeded DBUF_CTL_DIST and DBUF_CTL_ADDR
      macros. Started to use _PICK construct as suggested
      by Matt Roper.

v3: - _DBUF_CTL_S* to DBUF_CTL_S*, changed X to "slice"
      in macro(Ville Syrjälä)
    - Introduced enum for enumerating DBUF slices(Ville Syrjälä)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200202230630.8975-5-stanislav.lisovskiy@intel.com