drm/i915: Shuffle stride checking code around

drm/i915: Shuffle stride checking code around

Reorganize some fb stride checking code a bit to prepare for
gtt remapping. And do a bit of s/pitch/stride/ renaming in the
process for a bit more uniformity (apart from the whole
fb->pitches[] thing).

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190509122159.24376-5-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>