st/mesa: Prefer 5551 formats for GL_UNSIGNED_SHORT_5_5_5_1.

Kenneth Graunke requested to merge kwg/mesa:deqp-fix into master

Previously, internalformat GL_RGBA and type GL_UNSIGNED_SHORT_5_5_5_1 was promoted to RGBA8888 as the table entry with the 5551 formats is listed below the 8888 entry, and it also doesn't have GL_RGBA as a possible internalformat.

Using actual 5551 fixes the following dEQP-EGL test:

  • dEQP-EGL.functional.image.modify.tex_rgb5_a1_tex_subimage_rgba8

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