drm/i915: Fix fb object's frontbuffer-bits

Shared frontbuffer bits are causing warnings when same FB is displayed
in another plane without clearing the bits from previous plane.

v2: Removing coversion of fb bits to 64 bit as it is not needed for now. (Daniel)

Change-Id: Ic2df80747f314b82afd22f8326297c57d1e652c6
Signed-off-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Kumar, Mahesh <mahesh1.kumar@intel.com>
[danvet: Drop INTEL_FRONTBUFFER_SPRITE_MASK since unused.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>