mesa: Always use seamless cubemap filtering in GLES3

mesa: Always use seamless cubemap filtering in GLES3

Appendix F.2 of the OpenGL ES 3.0.0 spec says:

    "OpenGL ES 3.0 requires that all cube map filtering be
    seamless. OpenGL ES 2.0 specified that a single cube map face be
    selected and used for filtering."

Setting the field only in the context will work fine with sampler
objects (and drivers that support AMD_seamless_cubemap_per_texture)
because seamless filtering is used if *either* the context or the
sampler enable it:

    "If TEXTURE_CUBE_MAP_SEAMLESS_ARB is emabled (sic) globally or the
    value of the texture's TEXTURE_CUBE_MAP_SEAMLESS_ARB parameter is
    TRUE, seamless cube map sampling is enabled..."

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reported-by: Maxence Le Dore <maxence.ledore@gmail.com>
Thanked-by: Maxence Le Dore <maxence.ledore@gmail.com>