iris: Support sRGB fast clears even if the colorspaces differ.

iris: Support sRGB fast clears even if the colorspaces differ.

We were disabling fast clears if the view format had a different
colorspace than the resource format (sRGB vs linear or vice-versa). But
we actually support them if we use the view format to decide if we
should encode the clear color into sRGB colorspace.

Also add a missing linear -> sRGB surface format conversion (we don't
want the clear color to be encoded to sRGB again during resolve).

v2: Do not track sRGB colorspace during fast clears (Nanley).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>