Make things a bit more abstract by replacing the pch_pfit.pos/size
raw register values with a drm_rect. Makes it slighly more convenient
to eg. compute the scaling factors.

v2: Use drm_rect_init()
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200422161917.17389-3-ville.syrjala@linux.intel.comReviewed-by: Manasi Navare <manasi.d.navare@intel.com>