Due to a dEQP bug, EXT_texture_sRGB_R8 cannot be exposed before
EXT_texture_sRGB_decode is available. This change disables
EXT_texture_sRGB_R8 on the vulkan backend.

Bug: angleproject:3609
Test: dEQP-GLES31.functional.srgb_texture_decode.skip_decode.sr8.*
Change-Id: I261c80986484e489f6903eb54ac6865433050bfa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2161985Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>