drm/i915/ehl: Inherit Ice Lake conditional code

Most of the conditional code for ICELAKE also applies to ELKHARTLAKE
so use IS_GEN(dev_priv, 11) even for PM and Workarounds for now.

v2: - Rename commit (Jose)
    - Include a wm workaround (Jose and Lucas)
    - Include display core init (Jose and Lucas)
v3: Add a missing case of gen greater-than 11 (Jose)

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190412180920.22347-1-rodrigo.vivi@intel.com