drm/i915: Introduce intel_crtc_state_alloc()

We have several places where we want to allocate a pristine
crtc state. Some of those currently call intel_crtc_state_reset()
to properly initialize all the non-zero defaults in the state, but
some places do not. Let's add intel_crtc_state_alloc() to do both
the alloc and the reset, and call that everywhere we need a fresh
crtc state.

v2: s/kzalloc/kmalloc/ since we memset() anyway (José)

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191219111430.17527-1-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>