drm/i915: introduce REG_BIT() and REG_GENMASK() to define register contents

drm/i915: introduce REG_BIT() and REG_GENMASK() to define register contents

Introduce REG_BIT(n) to define register bits and REG_GENMASK(h, l) to
define register bitfield masks.

We define the above as wrappers to BIT() and GENMASK() respectively to
force u32 type to go with our register size, and to add compile time
checks on the bit numbers.

The intention is that these are easier to get right and review against
the spec than hand rolled masks.

Convert power sequencer registers as an example.

v4:
- rebase

v3:
- rename macros to REG_BIT() and REG_GENMASK() to avoid underscore
  prefix and to be in line with kernel macros (Chris)
- add compile time checks (Mika)

v2:
- rename macros to just _BIT() and _MASK() to reduce verbosity

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/787307c0ba9bc23471e5ff1e454b8af35771fa37.1552657998.git.jani.nikula@intel.com