glsl: Keep matrix inversion in highp

I _think_ there is a risk of overflow while calculating the 2x2
determinant a*d - b*c. The difference may fit in fp16 though the
individual products may not. The Mali blob promotes the products to
highp, doing the same fixes failures on Bifrost:

dEQP-GLES3.functional.shaders.matrix.inverse.dynamic.mediump_mat2_float_vertex
dEQP-GLES3.functional.shaders.matrix.inverse.dynamic.lowp_mat2_float_vertex

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
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