evas/gl_x11: set EGL_PLATFORM environment variable

evas/gl_x11: set EGL_PLATFORM environment variable

Summary:
Without EGL_PLATFORM environment variable, eglInitialize() can be
failed because egl tried to load DRM platform instead of X11 platform and it
tried to handle XDisplay pointer as a gbm_device pointer as well.
The failure seems to be occured especially if the egl was built
with DRM platform as native platform.

This revision can prevent the failure by indicating proper egl platform using
EGL_PLATFORM environment variable, and additionally it will unset
EGL_PLATFORM environment value when the engine free its info.

@fix

Reviewers: gwanglim, seoz, jaehwan, cedric, raster

Subscribers: raster, cedric

Differential Revision: https://phab.enlightenment.org/D1844