drm/i915: Assert that we don't overflow frontbuffer tracking bits

drm/i915: Assert that we don't overflow frontbuffer tracking bits

Add some compile time assrts to the frontbuffer tracking to make sure
that we have enough bits per pipe to cover all the planes, and that we
have enough total bits to cover all the planes across all pipes.

We'll ignore any potential clash between the overlay bit and the
plane bits because that will allow us to keep using a total of 32
bits for the foreseeable future.

While at it change the macros to use BIT() and GENMASK(). The latter
gets rid of the hardcoded 0xff and thus means we can change the
number of bits per pipe by just changing
INTEL_FRONTBUFFER_BITS_PER_PIPE.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180124183642.32549-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>