drm/i915: Refactor sseu helper functions

Move functions to intel_sseu.h and remove inline qualifier.
Additionally, ensure these are all prefixed with intel_sseu_*
to match the convention of other functions in i915.

v2: fix spacing from checkpatch warning
v3: squash helper function changes into a single patch
    break 80 character line to fix checkpatch warning
    move get/set_eus helpers to intel_device_info.c
v4: Remove intel_ prefix from static functions in
    intel_device_info.c and correctly copy changes
    to stride calculation in those functions.

Acked-by: Jani Nikula <jani.nikula@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190524154022.13575-5-stuart.summers@intel.com