st/mesa/i965: Allow decompressing ETC2 to GL_RGBA

st/mesa/i965: Allow decompressing ETC2 to GL_RGBA

When Mesa itself implements ETC2 decompression, it currently
decompresses to formats in the GL_BGRA component order.

That can be problematic for drivers which cannot upload the texture data
as GL_BGRA, such as Virgl when it's backed by GLES on the host.

So this commit adds a flag to _mesa_unpack_etc2_format so callers can
specify the optimal component order.

In Gallium's case, it will be requested if the format isn't in
PIPE_FORMAT_B8G8R8A8_SRGB format.

For i965, it will remain GL_BGRA, as before.

v2: * Remove unnecesary include (Emil Velikov)

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>