drm/i915/bdw: Broadwell has PIPEMISC

And it inherits some bits from the previous TRANS_CONF (aka PIPE_CONF
on previous gens).

v2: Rebase on to of the pipe config bpp handling rework.

v3: Rebased on top of the pipe_config->dither refactoring.

v4: Drop the read-modify-write cycle for PIPEMISC, similarly to how we
now also build up PIPECONF completely ourselves - keeping around
random stuff set by the BIOS just isn't a good idea. I've checked BDW
BSpec and we already set all relevant bits.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com> (v1)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>