drm/i915: Clean up device info structure definitions

Beginning with gen7, newer devices repetitively redefine values
for the device info structure members.  This patch simplifies the
structure definitions by grouping member value definitions into the
existing GEN7_FEATURES #define and into the new GEN7_LP_FEATURES
and HSW_FEATURES #defines.

Specifically, GEN_DEFAULT_PIPEOFFSETS and IVB_CURSOR_OFFSETS are
added to GEN7_FEATURES and subsequent IVB definitions are simplified.

VLV_FEATURES is defined to differentiate and simplify the
gen7 low power (LP) devices.

HSW_FEATURES is defined and used to simplify all HSW+ devices
except for LP.

v2: Use VLV_FEATURES for the gen7 low power devices. (Jani)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Wayne Boyer <wayne.boyer@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449091694-7681-1-git-send-email-wayne.boyer@intel.com