platforms/drm: Use a heuristic to determine if EGLDevice backend can be used

Currently, in order to use the EGLStreams backend, you need to set a
kernel parameter and an environment variable. If you set only the kernel
parameter, performance-wise, kwin will be unusable.

This change makes using the EGLStreams backend easier by making the env
var optional. If you omit KWIN_DRM_USE_EGL_STREAMS=1, kwin will check the
driver name to determine whether the EGLStreams can be enabled.