gpu-viv-bin-mx6q, mesa: fix virtual/libgl dependencies

gpu-viv-bin-mx6q, mesa: fix virtual/libgl dependencies

Fixes [YOCTO #4850]

findings:
* meta-fsl-arm/recipes-graphics/mesa/mesa_9.1.3.bbappend is used to
  delete libGL.* after compilation of mesa

* ./recipes-graphics/gpu-viv-bin-mx6q/gpu-viv-bin-mx6q_3.0.35-4.0.0.bb, via
  gpu-viv-bin-mx6q.inc, provides its own version of libGL

* gpu-viv-bin-mx6q does not advertise "virtual/libgl" in "PROVIDES +="

* but mesa (via mesa.inc) does:
  PROVIDES = "virtual/libgl virtual/libgles1 virtual/libgles2 virtual/egl"

* so the statement in imx-base.inc
   PREFERRED_PROVIDER_virtual/libgl_mx6 ?= "gpu-viv-bin-mx6q"
  does not have effect, because mesa is still the only provider
  of virtual/libgl

* packages depending on virtual/libgl only get mesa, which will not contain
  the libGL library

* in fact, we still use the gl headers from mesa, but the binary blob library
  from gpu-viv-bin-mx6q (gpu-viv-bin-mx6q does not provide the headers),
  so a package compiling against GL needs both mesa and gpu-viv-bin-mx6q

conclusion:
* so i suggest having gpu-viv-bin-mx6q depend on mesa to draw in the
  gl header package, advertising "virtual/libgl" for gpu-viv-bin-mx6q
  via "PROVIDES +=" and not advertising virtual/libgl for mesa via the
  .bbappend file

Signed-off-by: Simon Braunschmidt <sb@emlix.com>