drm/i915: Bump DSL linemask to 20 bits

drm/i915: Bump DSL linemask to 20 bits

Since tgl PIPE_DSL has 20 bits for the scanline. Let's bump our
definition to match. And while at it let's also add the define
for the current field readback.

We can also get rid of the gen2 vs. gen3+ nonsense since none
of the extra bits ever did anything and just always read
as zero. And now we extend all platforms to use the tgl+
20 bits deinition, but again that is fine since all the bits
used to be mbz and always read as zero on all the platforms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211112193813.8224-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>