i965: Pretend we don't support BRW_SURFACEFORMAT_R16G16B16_FLOAT for textures.

i965: Pretend we don't support BRW_SURFACEFORMAT_R16G16B16_FLOAT for textures.

None of the other 3-component 16bpc formats are directly supported, so
they get promoted to XRGB equivalents. *Not* promoting RGB16F the same
way makes texture views much more fiddly -- we don't want to have to do
crazy copying behind the scenes.

(with my other master + my experimental ARB_texture_view support) fixes
the piglit test: `spec/ARB_texture_view/view compare 48bit formats`

No regressions in gpu.tests on Haswell.

V4: Don't alter the formats table -- just don't match it to a mesa_format. [Kenneth]

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Eric Anholt <eric@anholt.net>