mesa/main: Use flag for EXT_sRGB instead of EXT_framebuffer_sRGB where possible

mesa/main: Use flag for EXT_sRGB instead of EXT_framebuffer_sRGB where possible

All drivers that support EXT_framebuffer_sRGB also support EXT_sRGB, but
in order to keep this commit minial, and not to break any drivers both
flags are checked.

v2: - Use only EXT_sRGB (Ilia Mirkin)
    - Move adding the flag EXT_sRGB to gl_extensions to a separate patch

v3: use _mesa_has_EXT_framebuffer_sRGB instead of extension flag
    The _mesa_has function also checks for the correct versions and
    should be preferred over using the flags directly (Erik)

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