st/mesa: Fall back to R8G8B8A8_SRGB for ETC2

st/mesa: Fall back to R8G8B8A8_SRGB for ETC2

If the driver doesn't support PIPE_FORMAT_B8G8R8A8_SRGB, fall back to
PIPE_FORMAT_R8G8B8A8_SRGB.

Drivers such as Virgl will have a hard time supporting
PIPE_FORMAT_B8G8R8A8_SRGB when the host runs GLES, as GL_BGRA isn't as
well suported there.

So go with PIPE_FORMAT_R8G8B8A8_SRGB so these drivers can avoid a
conversion copy.

v2: Fix typo in commit message

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