asahi: Make clear the non-sRGBness of EOT images

asahi: Make clear the non-sRGBness of EOT images

For sRGB render targets, we encode sRGB when writing pixels into the tilebuffer
(in the fragment shader), not when writing out the image. When we actually write
out the tilebuffer to the image, we don't use the PBE's sRGB conversion, we just
bind it as a UNORM 8 image and blit the pre-transformed pixels.

We're about to add real sRGB support for the PBE, so make this linearization
explicit.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24258>