drm/vram-helper: Remove invariant parameters from internal kmap function

The parameters map and is_iomem are always of the same value. Removed them
to prepares the function for conversion to struct dma_buf_map.

v4:
	* don't check for !kmap->virtual; will always be false

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201103093015.1063-2-tzimmermann@suse.de