glsl: do not call link_xfb_stride_layout_qualifiers() for fragment shaders

glsl: do not call link_xfb_stride_layout_qualifiers() for fragment shaders

xfb only applies to the latest stage before the fragment shader, so
there is no need to invoke it in the fragment shader.

Fixes:
KHR-GL45.enhanced_layouts.xfb_stride_of_empty_list
KHR-GL45.enhanced_layouts.xfb_stride_of_empty_list_and_api

v2: do reset only if shaders provide an explicit stride

v3: do not call link_xfb_stride_layout_qualifiers() for fragment shaders
(Timothy)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>