drm/i915: remove intel_update_linetime_watermarks

drm/i915: remove intel_update_linetime_watermarks

The spec says the linetime watermarks must be programmed before
enabling any display low power watermarks, but we're currently
updating the linetime watermarks after we call intel_update_watermarks
(and only at crtc_mode_set, not at crtc_{enable,disable}). So IMHO the
best way guarantee the linetime watermarks will be updated before the
low power watermarks is inside the update_wm function, because it's
the function that enables low power watermarks. And since Haswell is
the only platform that has linetime watermarks, let's completely kill
the "intel_update_linetime_watermarks" abstraction and just use the
intel_update_watermarks abstraction by creating haswell_update_wm.

For now haswell_update_wm is still calling sandybridge_update_wm, but
in the future I plan to implement a function specific to Haswell.

v2: - Rename patch
    - Disable LP watermarks before changing linetime WMs (Chris)
    - Add a comment explaining that this is just temporary code.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>