drm: avoid "possible bad bitmask?" warning

Recent versions of gcc say this:

include/drm/i915_drm.h:96:34: warning: result of ‘65535 << 20’
requires 37 bits to represent, but ‘int’ only has 32 bits
[-Wshift-overflow=]

Reported-by: David Binderman <[e-mail]>
Signed-off-by: Dave Gordon <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Reviewed-by: Chris Wilson <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]

include/drm/i915_drm.h

···
 #define    I845_TSEG_SIZE_1M	(3 << 1)
 
 #define INTEL_BSM 0x5c
-#define   INTEL_BSM_MASK (0xFFFF << 20)
+#define   INTEL_BSM_MASK	(-(1u << 20))
 
 #endif				/* _I915_DRM_H_ */