drm/i915: Remove unused BSM_MASK causing warning

Remove never used BSM{,_MASK}. BSM_MASK #define also causes a warning.

include/drm/i915_drm.h:96:34: warning: result of ‘65535 << 20’
requires 37 bits to represent, but ‘int’ only has 32 bits
[-Wshiftoverflow=]
   #define   INTEL_BSM_MASK (0xFFFF << 20)

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1476256734-6457-1-git-send-email-joonas.lahtinen@linux.intel.com