meson/vrend/vkr: clean up gbm requirement
Summary:
- first 2 commits re-tighten up gbm<->EGL dependency for vrend (overly loosened in bd854bf8)
- 3rd commit tightens up minigbm<->EGL dependency for vrend
- 4th commit makes gbm an optional dependency for venus only when minigbm is used
Tested with various meson setup option combinations.
Related issue: #336
Edited by Yiwei Zhang