drm/i915: fix build for CONFIG_BUG=n

drm/i915: fix build for CONFIG_BUG=n

If CONFIG_BUG=n __WARN_printf won't be defined leading to the below
build failure. The double underscores should have told us to steer clear
of it anyway.

drivers/gpu/drm/i915/intel_display.c: In function ‘assert_pll’:
drivers/gpu/drm/i915/intel_display.c:1027:2: error: implicit declaration
of function ‘__WARN_printf’ [-Werror=implicit-function-declaration]
  I915_STATE_WARN(cur_state != state,

Use WARN(1, ...) instead. It handles CONFIG_BUG=n gracefully and, with
the constant condition, a sane compiler should reduce it to
__WARN_printf.

This is a regression introduced by

commit bb0d923e0d8a3ba3735a12239a16b13f439e1301
Author: Rob Clark <robdclark@gmail.com>
Date:   Mon Dec 15 13:56:32 2014 -0500

    drm/i915: tame the chattermouth (v2)

Reported-by: Jim Davis <jim.epost@gmail.com>
Reference: http://mid.gmane.org/CA+r1ZhgHTi7bS2irhtuSUs9aO=Br1dumN8=oAOeaMJDZ_ZhwBw@mail.gmail.com
Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>