drm/i915: Only include i915_reg.h from .c files

Several of our i915 header files, have been including i915_reg.h.  This
means that any change to i915_reg.h will trigger a full rebuild of
pretty much every file of the driver, even those that don't have any
kind of register access.  Let's delete the i915_reg.h include from all
headers and add an explicit include from the .c files that truly
need the register definitions; those that need a definition of
i915_reg_t for a function definition can get it from i915_reg_defs.h
instead.

We also remove two non-register #define's (VLV_DISPLAY_BASE and
GEN12_SFC_DONE_MAX) into i915_reg_defs.h to allow us to drop the
i915_reg.h include from a couple of headers.

There's probably a lot more header dependency optimization possible, but
the changes here roughly cut the number of files compiled after 'touch
i915_reg.h' in half --- a good first step.

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220127234334.4016964-7-matthew.d.roper@intel.com