drm/i915: Move SNPS PHY registers to their own header

These registers are only needed in a couple files and on specific
platforms; let's keep them separate from the general register pool.

v2:
 - Don't forget to include i915_reg_defs.h (Jani)
 - Ensure include guard matches header name (Jani)

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/20220111051600.3429104-9-matthew.d.roper@intel.com