glx: Set sRGBCapable to a default value

glx: Set sRGBCapable to a default value

Previously, if the server didn't send a GLX_FRAMEBUFFER_SRGB_CAPABLE_EXT
tag, it would still be set to GLX_DONT_CARE (which is -1).  Set it to
GL_FALSE instead.

NOTE: This is a candidate for stable release branches.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: Maciej Wieczorek <maciej.t.wieczorek@intel.com>