drm/i915: Add intel_get_crtc_scanline()

Add a new function intel_get_crtc_scanline() that returns the current
scanline counter for the crtc.

v2: Rebase after vblank timestamp changes.
    Use intel_ prefix instead of i915_ as is more customary for
    display related functions.
    Include DRM_SCANOUTPOS_INVBL in the return value even w/o
    adjustments, for a bit of extra consistency.
v3: Change the implementation to be based on DSL on all gens,
    since that's enough for the needs of atomic updates, and
    it will avoid complicating the scanout position calculations
    for the vblank timestamps
v4: Don't break scanline wraparound for interlaced modes

Reviewed-by: Sourab Gupta <sourabgupta@gmail.com>
Reviewed-by: Akash Goel <akash.goels@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>