drm/i915/display: Split and export main surface calculation from skl_check_main_surface()

The calculation the offsets of the main surface will be needed by PSR2
selective fetch code so here splitting and exporting it.
No functional changes were done here.

v3: Rebased

Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Tested-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210104205654.238928-3-jose.souza@intel.com