nouveau: Fix non-1x post-scale factor with DOT3 combiner

nouveau: Fix non-1x post-scale factor with DOT3 combiner

Fixes long standing bug on NV10 and NV20 where using a non-1x RGB or A
post-scale with GL_DOT3_RGB or GL_DOT3_RGBA texture environment would
not work.

The old combiner math uses HALF_BIAS_NORMAL and HALF_BIAS_NEGATE.  The
GL_NV_register_combiners defines these as

    HALF_BIAS_NORMAL_NV       max(0.0, e) - 0.5
    HALF_BIAS_NEGATE_NV       -max(0.0, e) + 0.5

In order to get the correct result from the dot-product, the
intermediate dot-product must be multiplied by 4.  This is a literal
implementation of the GL_ARB_texture_env_dot3 spec.  It also requires
using the register combiner post-scale.  As a result, the post-scale
cannot be used for the post-scale set by the application.

The new combiner math uses EXPAND_NORMAL and EXPAND_NEGATE.  The
GL_NV_register_combiners defines these as

    EXPAND_NORMAL_NV          2.0 * max(0.0, e) - 1.0
    EXPAND_NEGATE_NV          -2.0 * max(0.0, e) + 1.0

Since this fully expands the value to [-1, 1] range, the intermediate
dot-product result is the desired value.  This leaves the register
combiner post-scale available for application use.

NOTE: I have not actually tested this.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>