egl/main: add support for fourth plane tokens

egl/main: add support for fourth plane tokens

The EGL_EXT_dma_buf_import_modifiers extension adds support for a
fourth plane, just like DRM KMS API does.

Bump maximum dma_buf plane count to four.

v2: prevent attribute tokens from being parsed if
    EXT_image_dma_buf_import_modifiers is not suported. (Emil Velikov)

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Varad Gautam <varad.gautam@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>