i965: Make sRGB-capable framebuffers by default.

i965: Make sRGB-capable framebuffers by default.

The GLX extension lets you expose visuals that explicitly guarantee you
that the GL_FRAMEBUFFER_SRGB_CAPABLE flag will be set, but we can set
the flag even while the visual doesn't provide the guarantee.  This
appears to be consistent with other implementations, as we've seen
several apps now that don't require an srgb visual and assume sRGB will
work without checking the GL_FRAMEBUFFER_SRGB_CAPABLE flag.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=55783
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=60633
Reviewed-and-tested-by: Ian Romanick <ian.d.romanick@intel.com>