drm/i915: Ensure intel_engine_init_execlist() builds with Clang

Clang build with UBSAN enabled leads to the following build error:

drivers/gpu/drm/i915/intel_engine_cs.o: In function `intel_engine_init_execlist':
drivers/gpu/drm/i915/intel_engine_cs.c:411: undefined reference to `__compiletime_assert_411'

Again, for this to work the code would first need to be inlined and then
constant folded, which doesn't work for Clang because semantic analysis
happens before optimization/inlining.

Use GEM_BUG_ON() instead of BUILD_BUG_ON().

v2: Use is_power_of_2() from log2.h (Chris)

References: http://mid.mail-archive.com/20181015203410.155997-1-swboyd@chromium.orgReported-by: NStephen Boyd <swboyd@chromium.org>
Cc: Stephen Boyd <swboyd@chromium.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: NNathan Chancellor <natechancellor@gmail.com>
Tested-by: NStephen Boyd <swboyd@chromium.org>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NNick Desaulniers <ndesaulniers@google.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181016122938.18757-2-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -466,7 +466,7 @@ static void intel_engine_init_execlist(struct intel_engine_cs *engine)
 	struct intel_engine_execlists * const execlists = &engine->execlists;
 
 	execlists->port_mask = 1;
-	BUILD_BUG_ON_NOT_POWER_OF_2(execlists_num_ports(execlists));
+	GEM_BUG_ON(!is_power_of_2(execlists_num_ports(execlists)));
 	GEM_BUG_ON(execlists_num_ports(execlists) > EXECLIST_MAX_PORTS);
 
 	execlists->queue_priority = INT_MIN;