i965: Set the FBO error state INCOMPLETE_ATTACHMENT only for SRGB_R8

i965: Set the FBO error state INCOMPLETE_ATTACHMENT only for SRGB_R8

Originally the driver reported GL_FRAMEBUFFER_UNSUPPORTED in all cases,
adding more specific error messages was not correct and broke many tests.
Mostly revert this and only report GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT
for MESA_FORMAT_R_SRGB8.

Fixes: ebcde3454552adc6d3fea8af2207aafaba857796
  i965: be more specific about FBO completeness errors

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108805

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>