drm/i915: Use intel_tile_height() instead of re-implementing

intel_tile_dims() computes tile height using size and width, when there
is already a function to do just that - intel_tile_height()

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191221120543.22816-2-imre.deak@intel.com