gallium: introduce format modifier querying

gallium: introduce format modifier querying

format modifiers tokens are driver specific, and hence, need to come
in from the driver. this allows drivers to be queried for supported
format modifiers for EGL_EXT_image_dma_buf_import_modifiers.

v2: rebase to master.
v3: drivers must return false on query failure.
v4: use pscreen->is_format_supported instead of adding a separate
    format query handle, remove PIPE_CAP_QUERY_DMABUF_ATTRIBS.
    (Lucas Stach)
v5: add external_only parameter.

Signed-off-by: Varad Gautam <varad.gautam@collabora.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>