st/dri: Don't expose sRGB formats to clients

Though the SARGB8888 format is used internally through its FourCC value,
it is not a real format as defined by drm_fourcc.h; it cannot be used
with KMS or other interfaces expecting drm_fourcc.h format codes.

Ensure we don't advertise it through the dmabuf format/modifier query
interfaces, preventing us from tripping over an assert.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reported-by: Michel Dänzer <michel.daenzer@amd.com>
Fixes: 8c1b9882 ("egl/dri2: Guard against invalid fourcc formats")
Acked-by: Jason Ekstrand <jason.ekstrand@intel.com>
(cherry picked from commit 01c0aa9f)