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.