glsl: fix assignment of multiple scalar and vecs to matrices.

When a vec has more elements than row components in a matrix, the
code could end up failing an assert inside assign_to_matrix_column().

This patch makes sure that when there is still room in the matrix for
more elements (but in other columns of the matrix), the data is actually
assigned.

This patch fixes the following dEQP test:

  dEQP-GLES3.functional.shaders.conversions.matrix_combine.float_bvec4_ivec2_bool_to_mat4x2_vertex

No piglit regressions observed

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>