drm: use kernel macros

/** \name Internal types and structures */

/*@{*/

#define DRM_ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))

#define DRM_MIN(a,b) ((a)<(b)?(a):(b))

#define DRM_MAX(a,b) ((a)>(b)?(a):(b))

#define DRM_ARRAY_SIZE(x) ARRAY_SIZE(x)

#define DRM_MIN(a,b) min(a,b)

#define DRM_MAX(a,b) max(a,b)

#define DRM_LEFTCOUNT(x) (((x)->rp + (x)->count - (x)->wp) % ((x)->count + 1))

#define DRM_BUFCOUNT(x) ((x)->count - DRM_LEFTCOUNT(x))