drm/i915/gen9: fix plane_blocks_per_line on watermarks calculations

The confusing thing is that plane_blocks_per_line is listed as part of
the method 2 calculation but is also used for other things. We
calculated it in two different places and different ways: one inside
skl_wm_method2() and the other inside skl_compute_plane_wm(). The
skl_wm_method2() implementation is the one that matches the
specification.

With this patch we fix the skl_compute_plane_wm() calculation and just
pass it as a parameter to skl_wm_method2(). We also take care to not
modify the value of plane_bytes_per_line since we're going to rely on
it having a correct value in later patches.

This should affect the watermarks for Linear and Y-tiled.

From my analysis, it looks like the two plane_blocks_per_line
variables got out of sync on 0fda65680e92, but we can't really say
that commit was a regression, it looks like just an incomplete fix.
There's always the possibility that 0fda65680e92 matched our
specification at that time, and then later the specification changed.

v2: Try to add a "Fixes" tag (Maarten).

Fixes: 0fda65680e92 ("drm/i915/skl: Update watermarks for Y tiling")
Cc: [email protected]
Cc: Tvrtko Ursulin <[email protected]>
Reviewed-by: Lyude <[email protected]>
Reviewed-by: Maarten Lankhorst <[email protected]>
Signed-off-by: Paulo Zanoni <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
(cherry picked from commit 7a1a8aed67e0a60772defe3f6499eb340da48634)
Signed-off-by: Jani Nikula <[email protected]>

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