drm/i915: make i915_stolen_to_physical() return phys_addr_t

The i915_stolen_to_physical() function has 'unsigned long' as its
return type but it returns the 'base' variable, which is of type
'u32'. The only place where this function is called assigns the
returned value to dev_priv->mm.stolen_base, which is of type
'phys_addr_t'. The return value is actually a physical address and
everything else in the stolen memory code seems to be using
phys_addr_t, so fix i915_stolen_to_physical() to use phys_addr_t.

v2: Add missing blank lines after declarations (Chris, checkpatch.pl).

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1485461947-16030-1-git-send-email-paulo.r.zanoni@intel.com