dri2: ignore BadDrawable errors.

dri2: ignore BadDrawable errors.

Hi,

This patch fixes the following error.

X Error of failed request:  BadDrawable (invalid Pixmap or Window parameter)
  Major opcode of failed request:  137 (DRI2)
  Minor opcode of failed request:  4 (DRI2DestroyDrawable)
  Resource id in failed request:  0x4800001
  Serial number of failed request:  30
  Current serial number in output stream:  32

The X drawable can die before we notice it in free_drawable_hashtable(),
called through vaTerminate(). This patch is ported from Mesa (GLX/DRI2).

Regards,
Gwenole.