mesa/st: rework support for sRGB framebuffer attachements

mesa/st: rework support for sRGB framebuffer attachements

For GLES sRGB framebuffer attachemnt support is provided in two steps:
sRGB attachments like described in EXT_sRGB (and GLES 3.0) that enable
linear to sRGB color space transformation automatically, and the ability
to switch formats of the render target surface between sRGB and linear
that introduces full support for EXT_framebuffer_sRGB.
Set the according flags to reflect these two levels of sRGB support.

As a difference between desktopm GL and GLES, on desktop GL for a sRGB
framebuffer attachment the linear-sRGB conversion is turned off by default,
and for GLES it is turned on. This needs to be taken into account when
initally creating a surface, i.e. on desktop GL creation of a sRGB surface
is preferred, but on GLES sRGB surfaces are only created when explicitely
requested.

v2: - Use the new CAPS name

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>