drm/i915/uncore: Convert gen6/gen7 read operations to fwtable

On gen6-gen8 (except vlv/chv) we don't use a forcewake lookup table; we
simply check whether the register offset is < 0x40000, and return
FORCEWAKE_RENDER if it is.  To prepare for upcoming refactoring, let's
define a single-entry forcewake table from [0x0, 0x3ffff] and switch
these platforms over to use the fwtable reader functions.

v2:
 - Drop __gen6_reg_read_fw_domains which is no longer used.  (Tvrtko)

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210910201030.3436066-2-matthew.d.roper@intel.com