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 <[email protected]>
Reported-by: Uma Shankar <[email protected]>
Fixes: baa09e7d2f42 ("drm/i915: use REG_FIELD_PREP() to define register bitfield values")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: Imre Deak <[email protected]>
Cc: Randy Dunlap <[email protected]>
Acked-by: Randy Dunlap <[email protected]> # build-tested
Reviewed-by: Jani Nikula <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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