gl-renderer: always add DRM_FORMAT_MOD_INVALID to EGL formats

EGL implementations have no way to tell that implicit modifiers are not
supported. So Weston must consider that implicit modifiers are
supported. Always add DRM_FORMAT_MOD_INVALID to formats that we query
from EGL.

The implication is that clients using dmabuf extension may pick
DRM_FORMAT_MOD_INVALID to allocate their buffers, and so these buffers
will not be directly imported to KMS and placed in planes. See commit
"backend-drm: do not import dmabuf buffers with no modifiers to KMS" for
more details.

But we should not avoid advertising DRM_FORMAT_MOD_INVALID in the dmabuf
protocol just because we hope that the client don't choose it, it's not
our choice.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>