intel: Add flags to intel_screen for hiz and separate stencil

intel: Add flags to intel_screen for hiz and separate stencil

Add the fields below to intel_screen. The expression in parens is the
value to which intelInitScreen2() currently sets the field.
    GLboolean hw_has_separate_stencil      (true iff gen >= 7)
    GLboolean hw_must_use_separate_stencil (true iff gen >= 7)
    GLboolean hw_has_hiz                   (always false)
    enum intel_dri2_has_hiz dri2_has_hiz   (INTEL_DRI2_HAS_HIZ_UNKNOWN)

The analogous fields in intel_context now inherit their values from
intel_screen.

When hiz and separate stencil become completely implemented for a given
chipset, then the respective fields need to be enabled.

CC: Ian Romanick <idr@freedesktop.org>
CC: Kristian Høgsberg <krh@bitplanet.net>
Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad@chad-versace.us>