drm/vmwgfx: Fix src/dst_pitch confusion

The src/dst_pitch got mixed up during the rework of the function, make
sure the offset's refer to the correct one.

Spotted by clang:
Clang warns (or errors with CONFIG_WERROR):

drivers/gpu/drm/vmwgfx/vmwgfx_stdu.c:509:29: error: variable 'dst_pitch' is uninitialized when used here [-Werror,-Wuninitialized]
src_offset = ddirty->top * dst_pitch + ddirty->left * stdu->cpp;
^~~~~~~~~
drivers/gpu/drm/vmwgfx/vmwgfx_stdu.c:492:26: note: initialize the variable 'dst_pitch' to silence this warning
s32 src_pitch, dst_pitch;
^
= 0
1 error generated.

Signed-off-by: Zack Rusin <[e-mail]>
Reported-by: Nathan Chancellor <[e-mail]>
Reported-by: Dave Airlie <[e-mail]>
Link: https://github.com/ClangBuiltLinux/linux/issues/1811
Fixes: 39985eea5a6d ("drm/vmwgfx: Abstract placement selection")
Reviewed-by: Nathan Chancellor <[e-mail]>
Reviewed-by: Martin Krastev <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/vmwgfx/vmwgfx_stdu.c

···
 	/* Assume we are blitting from Guest (bo) to Host (display_srf) */
 	src_pitch = stdu->display_srf->metadata.base_size.width * stdu->cpp;
 	src_bo = &stdu->display_srf->res.guest_memory_bo->tbo;
-	src_offset = ddirty->top * dst_pitch + ddirty->left * stdu->cpp;
+	src_offset = ddirty->top * src_pitch + ddirty->left * stdu->cpp;
 
 	dst_pitch = ddirty->pitch;
 	dst_bo = &ddirty->buf->tbo;
-	dst_offset = ddirty->fb_top * src_pitch + ddirty->fb_left * stdu->cpp;
+	dst_offset = ddirty->fb_top * dst_pitch + ddirty->fb_left * stdu->cpp;
 
 	(void) vmw_bo_cpu_blit(dst_bo, dst_offset, dst_pitch,
 			       src_bo, src_offset, src_pitch,