kms: hardcoded cursor size leading to artifacts

In the constructor QKmsCursor::QKmsCursor() there is a call to gbm_bo_create with a hardcoded width/height of 64x64.

We have a unit with a hardware cursor of 128x128. On this unit showing a Qt cursor leads to artifacts similar to: https://www.libreoffice.org/bugzilla/attachment.cgi?id=100690

I don't know much about drm etc. but I have just followed the following change from an xorg driver:
http://cgit.freedesktop.org/xorg/driver/xf86-video-modesetting/commit/?id=677935b2d20f54f21c645b5eb386b6c9485fee5f
and came up with a patch to Qt (see attachment) which seems to solve the problem for us. For example with this patch at the above-mentioned unit the cursor size is automatically recognized as 128x128 and I see no artifacts, just the cursor (e.g. an arrow).

In this patch I do the following:

• if environment variables
  QT_CURSOR_WIDTH
  QT_CURSOR_HEIGHT
  are set, then these values are used for cursor size (so if something goes
  wrong with drmGetCap you can always set size manually);

• if they are not set, then I call the drmGetCap() call to get settings and
  use these;
  (it's probably not needed: but as an extra protection: if I get from drmGetCap a value smaller than 32 then I use 32 instead

• if QT_CURSOR_WIDTH/HEIGHT is not set, and drmGetCap() fails then default
  value of 64 (as hardcoded in current qt sources) is used