drm/i915: Use parameterized GPR register definitions everywhere

Since we have an engine-parameterized macro GEN8_RING_CS_GPR, let's use
that in place of the HSW_CS_GPR and BCS_GPR register definitions.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220111051600.3429104-2-matthew.d.roper@intel.com