drm/i915: Move some variables to tighter scope

drm/i915: Move some variables to tighter scope

Clean up skl_allocate_pipe_ddb() a bit by moving the 'wm' variable
to tighter scope. We'll also consitify it where appropriate.

Also initialize plane_alloc/uv_plane_alloc when decrlaring them
rather than later.

v2: Update commit message (Matt)

Cc: Neel Desai <neel.desai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190312205844.6339-8-ville.syrjala@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>