i965/miptree: Store fast clear colors in an isl_color_value

i965/miptree: Store fast clear colors in an isl_color_value

This commit, out of necessity, makes a number of changes at once:

 1) Changes intel_mipmap_tree to store the clear color for both color
    and depth as an isl_color_value.

 2) Changes the depth/stencil emit code to do the format conversion of
    the depth clear value on Haswell and earlier instead of pulling a
    uint32_t directly from the miptree.

 3) Changes ISL's depth/stencil emit code to perform the format
    conversion of the depth clear value on Haswell and earlier instead
    of assuming that the depth value in the float is pre-converted.

 4) Changes blorp to pass the depth value through as a float.

 5) Changes the Vulkan driver to pass the depth value to blorp as a
    float rather than a uint.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chad Versace <chadversary@chromium.org>