nir/linker: Fix TRANSFORM_FEEDBACK_BUFFER_INDEX

nir/linker: Fix TRANSFORM_FEEDBACK_BUFFER_INDEX

From the ARB_enhanced_layouts specification:

  "For the property TRANSFORM_FEEDBACK_BUFFER_INDEX, a single integer
   identifying the index of the active transform feedback buffer
   associated with an active variable is written to <params>.  For
   variables corresponding to the special names "gl_NextBuffer",
   "gl_SkipComponents1", "gl_SkipComponents2", "gl_SkipComponents3",
   and "gl_SkipComponents4", -1 is written to <params>."

We were storing the xfb_buffer value, instead of the value
corresponding to GL_TRANSFORM_FEEDBACK_BUFFER_INDEX.

Note that the implementation assumes that varyings would be sorted by
offset and buffer.

Signed-off-by: Antia Puentes <apuentes@igalia.com>
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>