drm/i915/tgl: Make sure a semiplanar UV plane is tile row size aligned

Currently the GGTT offset of a UV plane in a semiplanar YUV FB is tile
size (4kB) aligned. I noticed, that enforcing only this alignment leads
oddly to random memory corruptions on TGL while scanning out Y-tiled
FBs. This issue can be easily reproduced with a UV plane offset that is
not aligned to the plane's tile row size.

Some experiments showed the correct alignment to be tile row size
indeed. This also makes sense, since the de-tiling fence created for the
object - with its own stride and so "left" and "right" edge - applies to
all the planes in the FB, so each tile row of all planes should be tile
row aligned.

In fact BSpec requires this alignment since SKL. On SKL we may enforce
this due to the AUX plane x,y coords check, but on ICL and TGL we don't.
For now enforce this only on TGL; I can follow up with any necessary
change for ICL after more tests.

BSpec requires a stricter alignment for linear UV planes too (kind of a
tile row alignment), but it's unclear whether that's really needed
(couldn't be explained with the de-tiling fence as above) and enforcing
that could break existing user space; so avoid that too for now until
more tests.

v2:
- Clarify the commit log wrt. the address space the alignment applies to.
  (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191231233756.18753-3-imre.deak@intel.com

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