drm/i915: Fix max cursor size for i915g/gm

drm/i915: Fix max cursor size for i915g/gm

Apparently the 128x128 and 256x256 ARGB cursor modes were
only added on LPT/CST.

While the display section of bspec isn't super clear on the
subject, it does highlight these two modes in a different
color, has a few changlog entries indicating the 256x256 mode
was added for a LPT DCN, and that the 128x128 mode was also
added later (though no DCN/platform note there).

The "device dependencies" bspec section does list the 256x256x32
as a new feature for LPT/CST, and goes on to mention that current
hw only has the 64x64x32 mode (which reinforces the notion that
the 128x128 mode was also added at the same time).

Testing on actual hardware confirms all of this. CI shows all
the 128x128 and 256x256 tests failing on GDG, and my ALV
definitely doesn't like them.

So we shall limit GDG/ALV to 64x64 only. And while at it
let's adjust the mobile gen2 case to list the two platforms
explicitly so that the if-ladder looks reasonably uniform.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191028113036.27553-2-ville.syrjala@linux.intel.com
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>