drm/i915/gen9: Disable WM if corresponding latency is 0

According to updated BSpec, If level 1 or any higher level has a value of 0x00,
that level and any higher levels are unused and the associated watermark
registers must not be enabled.

This patch checks for latency 0 for level >=1 and does not enable WM
corresponding to level m | m>=n, if level n (n != 0) has a 0us latency.

v2: Satheesh's review comments
	- zero-out latency values (for all higher levels if latency of given
	level is zero ) in read_wm_latency() function itself

v3: removed redundant check as per Satheesh's observation.
v4: rebase on top before merging (Damien)
v5: Rebase on top of the default value removal (Ville)

Reviewed-by: Satheeshakrishna M <satheeshakrishna.m@intel.com> (v3)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>