drm/i915: Use __is_constexpr()

gcc-4.8 and older dislike the use of __builtin_constant_p() within a
constant expression context, and so we must use the magical
__is_constexpr() instead.

For example, with gcc-4.8.5:
../drivers/gpu/drm/i915/i915_reg.h:167:27: error: first argument to ‘__builtin_choose_expr’ not a constant
../include/linux/build_bug.h:16:45: error: bit-field ‘<anonymous>’ width not an integer constant

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Uma Shankar <uma.shankar@intel.com>
Fixes: baa09e7d

 ("drm/i915: use REG_FIELD_PREP() to define register bitfield values")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190320154021.5244-1-chris@chris-wilson.co.uk