meta: Disable dithering during glGenerateMipmap

Fixes tests 'dEQP-GLES3.functional.texture.mipmap.*.generate.rgba5551*' on
Intel Broadwell 0x1616.

The GL 4.5 spec describes the algorithm of glGenerateMipmap as:

    The contents of the derived images are computed by repeated, filtered
    reduction of the level base image.  [...] No particular filter algorithm is
    required, though a box filter is recommended as the default filter.

Consider a texture for which all pixels are identical at level 0.
From the spec's description above, one may reasonably assume that the "filtered
reduction" of level 0 produces a new miplevel for which again all pixels are
identical. For any 2x2 subspan of identical pixels, it is difficult to see how
the "filtered reduction" of that subspan can produce a pixel that differs from
the source pixels.

Dithering during _mesa_meta_GenerateMipmap() violated that reasonable
assumption.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99210
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: mesa-stable@lists.freedesktop.org

src/mesa/drivers/common/meta_generate_mipmap.c

@@ -182,6 +182,7 @@ _mesa_meta_GenerateMipmap(struct gl_context *ctx, GLenum target,
 
    _mesa_meta_begin(ctx, MESA_META_ALL & ~MESA_META_DRAW_BUFFERS);
    _mesa_ColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
+   _mesa_Disable(GL_DITHER);
 
    /* Choose between glsl version and fixed function version of
     * GenerateMipmap function.