drm/i915: Determine I915_MAX_PLANES from plane enum

Let the compiler figure out what I915_MAX_PLANES is from 'enum plane' so
that we don't need a separate #define.

While we're at it, add the cursor plane to the enum.  This will cause
I915_MAX_PLANES to now include the cursor plane in its count (it didn't
previously).   This change is safe since we currently only use this
value in array declarations (never in the actual code logic); we just
wind up allocating slightly more memory than we need to.  A followup
patch will cause various parts of the code to start using the extra
array element where appropriate.

(This patch probably should have been squashed with the followup patch,
but I couldn't figure out how to get Coccinelle to modify enum
declarations...)

Suggested-by: Ander Conselvan De Oliveira <conselvan2@gmail.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>