doc: drm: Update references to previously renamed files

Update references to reservation.c and reservation.h since these files
have been renamed to dma-resv.c and dma-resv.h respectively.

Cc: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/323401/?series=65037&rev=1Signed-off-by: Anna Karas <anna.karas@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927111504.20136-1-anna.karas@intel.com

Documentation/driver-api/dma-buf.rst

@@ -118,13 +118,13 @@ Kernel Functions and Structures Reference
 Reservation Objects
 -------------------
 
-.. kernel-doc:: drivers/dma-buf/reservation.c
+.. kernel-doc:: drivers/dma-buf/dma-resv.c
    :doc: Reservation Object Overview
 
-.. kernel-doc:: drivers/dma-buf/reservation.c
+.. kernel-doc:: drivers/dma-buf/dma-resv.c
    :export:
 
-.. kernel-doc:: include/linux/reservation.h
+.. kernel-doc:: include/linux/dma-resv.h
    :internal:
 
 DMA Fences