intel: Add flags to intel_context for hiz and separate stencil

intel: Add flags to intel_context for hiz and separate stencil

Add the following flags:
    intel_context.has_separate_stencil
    intel_context.must_use_separate_stencil
    intel_context.has_hiz

The flags are currently set to false, and will be enabled for a given
chipset once the feature is completely implemented.

Since it may be some time before these features are completed, their
values can be overridden with environment variables INTEL_HIZ and
INTEL_SEPARATE_STENCIL. Valid values for these environment variables are
"0" and "1".

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad@chad-versace.us>